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Reloading Corner

RELOADERS CORNER: Standard Deviation

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Before getting into improving bullet velocity Standard Deviation, it’s first necessary to understand what it is, and what it isn’t. KEEP READING

chronograph screen

Glen Zediker

I got started on this topic last time, and kind of came in through the side door. Quick backstory: the topic was how to start on solving unsuitably high shot-to-shot velocity inconsistencies. This time we’ll start at the other end of this, and that is taking steps to improve already suitable velocity deviation figures.

Clearly, the first step in getting involved in velocity studies is getting the velocities to study. Of course, that means you need a chronograph. Midsouth Shooters has a selection and there’s a direct link in this article.

Virtually all chronographs are going to be accurate. A well-known manufacturer of shooting-industry electronics once told me that unless a chronograph displays a reading that’s just crazy unrealistic, you can rely on the number. The reason is that the current state of circuitry is pretty well understood and heavily shared. Pay attention, though, to setting up the device according to suggestions in the instructions that will accompany the new chronograph. The more recent Doppler-radar-based units are not technically chronographs, but they function as such. The advantages to those are many! More in another article soon. For now, for here, what matters is getting some numbers.

labradar
Latest and greatest, in my mind, advancement in data collection is doppler radar based units, like this from LabRadar. Easy to use, and not finicky about sunlight and setup.

Point of all that was this: You don’t have to spend up for the best to get a good chronograph. One of the price-point differences in chronographs is how much it will help work with the data it gathers. Most of us any more don’t have to do hands-on calculations. Me? All I want is a number. However, there are a good many that will record, calculate, and print.

magnetospeed
Barrel-mounted electro-magnetic chronographs like this one from MagnetoSpeed make it easy. I like being able to read speeds without all the setup, and not having to rely on a benchrest-type restriction. It stays on the rifle so can easily be used in the field. There are rail mounts available also.

Terms and Twists
Speaking of calculations, the most known and probably most used expressed calculation of collected velocity figures is Standard Deviation. SD suggests or reflects the anticipated consistency of bullet velocities (calculated from some number of recorded velocities). “Standard” reflects on a sort of an average of the rounds tested. I know saying “sort of” disturbs folks like my math-major son so here’s more: SD is the square root of the mean of the squares of the deviations.

Standard Deviation calculations did not originate from ballistic research. It’s from statistical analysis and can be applied to a huge number of topics, like population behavior. SD calculation forms a bell curve, familiar to anyone who ever had to take a dreaded Statistics class. The steeper and narrower the apex of the bell, the narrower the fluctuations were. But there’s always a bell to a bell curve and the greatest deviations from desired standard are reflected in this portion of the plot. Depending on the number of shots that went into the SD calculation, these deviations may be more or less notable than the SD figure suggests.

Calculating SD
If you have no electronic gadgetry to help: add up all the recorded velocities and divide them by the number of records to get a “mean.” Then subtract that mean value from each single velocity recorded to get a “deviation” from the mean. Then square each of those. Squaring them eliminates any negative numbers that might result from cancelling out and returning a “0.” Add the squares together and find the mean of the squares by dividing again by the number of numbers — minus 1 (divide by n -1; that eliminates a bias toward a misleadingly small result). Then find the square root of that and that’s the Standard Deviation figure, which is “a” Standard Deviation, by the way, not the Standard Deviation.

bell curve
This is a bell curve such as results from plotting an SD calculation, and is given here only an example of how the distribution, the “odds,” graph out.

Knowing a load’s SD allows us to estimate-anticipate how likely it is for “outliers” to show up as we’re shooting one round after another. Based on the distribution based on the curve, if we have an SD of 12, for instance, then a little better than 2 out of 3 shots will be at or closer to the mean than 12 feet per second (fps). The other shots will deviate farther: about 9 out of 10 will be 19 fps, or less, from the mean. 21 out of 22 will be 24 fps closer to the mean. Those numbers represent about 1.00, 1.65, and 2.00 standard deviations.

Now. All that may have ranged from really boring to somewhat helpful, to, at the least, I hope informative.

Mastery of SD calculation and understanding doesn’t necessarily mean smaller groups. It gives a way to, mostly and above all else, tell us, one, the potential of the ammo to deliver consistent elevation impacts, and, two, reflects on both how well we’re doing our job in assembling the ammo and the suitability of our component combination.

I honestly pay zero attention to SD. I go on two other terms, two other numbers. One is “range,” which is the lowest and highest speeds recorded in a session. The one that really matters to me, though, is “extreme spread.” That, misleading on the front end, is defined as the difference between this shot and the next shot, and then that shot and the next shot, and so on. Why? Because that’s how I shoot tournament rounds! This one, then another, and then another. A low extreme spread means that the accuracy of my judgment of my wind call has some support.

Depending on the number of shots and more, SD can be misleading because it gets a little smaller with greater amounts of input. Extreme spread doesn’t. I have yet to calculate an SD that put its single figure greater than my extreme spread records.

Lemmeesplain: The shot-to-shot routine is to fire a round. It’s either centered or not. If it’s not centered, calculate the amount of correction to get the next one to center. Put that on the sight. Fire again. If I know that there’s no more than 10 fps between those rounds, that’s no enough to account for (technically it can’t be accounted for with a 1/4-MOA sight) then it’s all on me, and if it’s all on me I know that the input I got from the last shot, applied to the next shot, will be telling. Was I right or wrong? It can’t be the ammo, folks. Then I know better whether the correction is true and correct.

Some might be thinking “what’s the difference?” and it’s small, and so are scoring lines.

A load that calculates to a low SD is not automatically going to group small, just because it has a low SD. Champion Benchrest competitors have told me that their best groups don’t always come with a low-SD load. But that does not apply to shooting greater distance! A bullet’s time of flight and speed loss are both so relatively small at 100 yards that any reasonable variation in bullet velocities (even a 20 SD) isn’t going to open a group, not even the miniscule clusters it takes to be competitive in that sport. On downrange, though, it really starts to matter.

For an example from my notes: Sierra 190gr .308 MatchKing, in a .308 Win. Its 2600 fps muzzle velocity becomes 2450 at 100 yards and 1750 at 600 yards (I rounded these numbers).

If we’re working with a horrid 100 fps muzzle velocity change, that means one bullet could lauch at 2550 and the next might hit 2650, in the extreme. The first drifts about 28 inches (let’s make it a constant full-value 10-mph wind again to keep it simple) and the faster one slides 26 inches. That’s not a huge deal. However! Drop — that is THE factor, and here’s where inconsistent velocities really hurt. With that 190, drop amount differences over a 100 fps range are about 3 times as great as drift amounts. This bullet at 2600 muzzle velocity hits 5-6 inches higher or lower for each 50 fps muzzle velocity difference. That is going to cost on target. And it gets way (way) worse at 1000 yards. Velocity-caused errors compound on top of “normal” group dispersion (which would be group size given perfect velocity consistency).

This 100 fps example is completely extreme, but half of that, or even a quarter of that, still blows up a score, or creates a miss on an important target.

That all led to this: What is a tolerable SD?

I say 12. There has been much (a huge amount) of calculation that led to that answer. But that’s what I say is the SD that “doesn’t matter” to accuracy. It’s more than I’ll accept for a tournament load, but for those I’m looking for an extreme spread less than 10 fps (the range might be higher, but now we’re just talking terms). More later…

Check out chronographs HERE

This article is adapted from Glen’s book, Handloading For Competition, available at Midsouth HERE. For more information on that and other books by Glen, visit ZedikerPublishing.com

Reloading Corner

RELOADERS CORNER: Velocity Consistency, Part One

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Shot-to-shot muzzle velocity consistency is almost always a high-ranking goal for the handloader. But what about when it’s just awful? READ MORE

chronograph display

Glen Zediker

Last edition the topic was a wide-scope look at propellants, and the underlying point was how to get started, how to choose one. There’s not a perfect answer to that, or not one I can warrant as absolutely decisive.

Propellant choice often comes down to experience (good and bad), and that’s one reason that many of us, and me most definitely, tend to stick with a few, and those also are the first we’ll try when starting up with a new project. It’s also one reason we might be hesitant to try a propellant again if it didn’t work well the last time. I have those hesitations.

There are also criteria that we’d all like to have met, and, as also said last time, sometimes those have to be ranked or weighted. We may not find the maximum velocity with the smallest group size with one propellant, and, for me, group size gets the most weight. That’s why I said that the best choice is often the one with the fewest compromises, and that’s assuming there’s likely to be some compromise, somewhere. And that’s a fair and wise assumption.

One criteria that I and others have pretty high on our lists is velocity consistency. One measure of a “good load” is low variations in measured muzzle velocities. This, without a doubt, is of more importance the more distant the target.

The propellant that tested showing the lowest shot-to-shot velocity deviation does not necessarily mean that load combination is going to be the most accurate. One reason it’s important might not only to do with on-target accuracy as it does with providing clues about either the handloading protocols we’re following or the suitability of the component combination we’re using.

This article will focus more on that last — suitability of the component combination — and more to follow later will be dedicated to the performance component of consistent velocities.

I got a letter just before doing this article asking about reasons for seeing high velocity deviations. This fellow, a loyal reader of my books, was using the same component combinations and tooling advice I take myself and also publish, and not getting good results. As a matter of fact, his results were horrid. He was seeing deviations, shot-to-shot, in the vicinity of 100 feet per second (fps), plus. That’s huge.

After much time spent testing all this to collect enough notebook entries to think I have some handle on it, a half grain (0.50 gr.) of propellant in most small- to medium-capacity cases (say from .223 Rem. to .308 Win.) is worth about 40 fps. Given that, 100 fps difference is not likely to come from a propellant charge level variance.

Another reader posted a comment-question last article here regarding how to know if aged components were still good, still performing as they should, and this is a place to start looking if we’re seeing radical inconsistencies.

Two questions at the same time, as I’ve said before, usually point me toward a topic.

Moisture is the enemy in propellant and primer storage. The “cool dry place” is hard to come by, around these parts anyhow. I’ve had propellant go bad after having been stored in resealed containers. So far, I haven’t had any lose its potency after many years of storage in the factory-sealed containers.

“Go bad” can mean at a couple of things, by the way. One is that the propellant ages to the point that it changes. If propellant “spoils” it smells bad! It will have an acrid aroma. Don’t use it. Another way it goes bad is pretty easy to tell: it clumps. That is too much moisture. Don’t use it. Put it out in the garden, it’s a great fertilizer — honest.

Primers? It’s hard to tell… Bad primers still appear good.

My letter-writer’s huge velocity deviations were solved by a change of primer, and, mostly, a box of fresh primers. I kind of knew that was the component-culprit because he was having the same results or effects from different propellants.

Primers should be stored in air-tight containers, which will be something other than the factory packaging. Primers are “sealed” but that’s a lightweight assurance. Touching them, for instance, won’t hurt them, contrary to rumors, but more prolonged exposure to excessive moisture can and will take a toll, and its effects are very likely to be as inconsistent as the performance of the compromised primers.

Another strong caution: Always remove, or never leave, however you prefer, propellant in a meter. After you’re done with the loading for the day, return it to its storage container and cap it back tightly. Same with primers. Any left over in the priming tube or tray should go back to safe storage. Clearly, this all has a lot to do with the environmental conditions of your loading-storage area.

Out of curiosity, I filled a case with some small-grained extruded propellant and left it sit out in my shop. It was clumped when I checked it next day (24 hours). I had to get a pipe cleaner (nearest handy tool) to get it all out of the case. I don’t store propellant or primers in my shop, and that’s the reason… Yes, we have some humidity in my part of the world.

Excluding those obvious issues, what makes some combinations produce higher or lower velocity consistencies takes some experimentation to improve (or give up on).

Sometimes (many times) this all seems more like art than science. It is science, of course, but it’s not tidy; it can’t always, or even often, be forecast.

I’ve seen the biggest effect from a primer brand change. I also, though, don’t swap primer brands around each time I do a load work up and the reason is that there are other attributes I need from a primer. Since I’m loading nearly always for a semi-auto, an AR15 specifically, I have to use a “tough” primer, and that also means one that will accept near-max pressure without incident.

Point is that if you’re running a rifle/ammo combination that isn’t limited by either propellant choice or primer choice, you might very well see some influential improvements by trying a different primer (after getting the propellant decided on). Do, always, reduce the charge at least a half grain before using a different primer brand — primer choices also decidedly influence velocity and pressure levels. Again, in my experience, more than you might imagine.

Next time, more about the performance component of consistent velocities, and a whopping lot more about how to improve that.

Check Midsouth storage solutions HERE

The preceding is a specially-adapted excerpt from Glen’s book Top-Grade Ammo. Available HERE at Midsouth Shooters Supply. Visit ZedikerPublishing.com for more information on the book itself, and also free article downloads.

Reloading Components, Reloading Corner, Uncategorized

RELOADERS CORNER: SD Pt. 2

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Here’s how Standard Deviation calculations can figure in ammo decisions (or not…) READ MORE…

Glen Zediker

Seems like the last couple of articles on load testing and velocity data got some pretty good responses and attention, and so that means there’s more! Of course there is…

As said, Standard Deviation (SD) plotted out forms a bell curve. A bell curve indicates the “probability density” of the normal distribution, or range, for something like velocity consistencies. For our purposes that’s the likely speed of the next shot.

Chances are outstanding that running all the numbers gotten from a chronograph session will plot into what’s called a “normal curve.” Like any normal bell curve, it gets divided into three segments and values, and these divisions are the “standard deviations.” And remember it is “a” standard deviation.

(I’ve said many a time that I’m sho no mathematician, and I am aware that there’s more and different ways to apply and model a curve, and to manipulate standard deviation results for different applications, but I’m trying to keep it more simple and use this “normal curve” for examples, it’s also called “population standard deviation.”)

We’ve been working with the right-respectable SD example of 12.

standard deviation curve
Here’s the same old curve I’ve been using, but at least in a different color!

Assuming that normal curve, the distribution of “some number” of shots is forecasted like so: some 68 percent will lie within 1 standard deviation of the mean, about 95 percent lie within 2, and over 99 percent lie within 3 standard deviations. Again, since our SD is 12, then about 68 percent (approx. 2 out of 3) of all “next shots” will be +/- 12 feet per second. Since, though, the curve is in threes, that means that a scant number of the shots pose a chance for +/- 24 and some much (much) smaller chance remains for some shots to go to +/- 36. SD estimates how likely it is for those “head-scratchers” to show up, and also what might be the most realistic extreme any shot can deviate.

Data is a record of numbers and I do know that there’s 100-percent chance that the highest and lowest velocities collected for an SD calculation did, in fact, happen. To me, that’s what matters. No matter what the collected shot results calculated into for an SD, those were the two that represent the highest and lowest prints on the target.

It’s mathematically not possible for an SD to be higher than the greatest single measured deviant, and an SD can sho be lower than any single “bad” shot. Given how it’s calculated, along with how many samples contributed to the calculation, it’s plain that the nearer the majority are to themselves the less impact a bad one or more has. The more input the better.

ppc
Cartridge choice has a whopping lot to do with it! Some cartridges are seemingly destined (designed really) to produce better velocity consistency. Many magnums, for instance, are notoriously sporadic, while others, like the 6XC or one of the PPC cartridges (shown), seem to deliver constant velocities without a lot of special effort. It all has to do with internal ballistics and “efficiency,” and architectural analysis I don’t claim to understand, but I do know that’s one of the reasons 6XC holds the NRA High Power Rifle Long Range record, at the hands of David Tubb.

Many of us have heard or read the frequently-sung “…seen good accuracy with high SDs…” And we’ve probably also all decided that can’t be taken at literal value. Well, it can’t. Three things: what is “good accuracy” to this fellow, at which distance were the groups printed, and what’s he say is “high,” because without knowing these things there’s no accounting for the accuracy, believability, or interpretative definitiveness of what’s being said. So I say it’s 12. A 12 should not be responsible for a points loss, also considering the edge limits of usual group size. Getting into more and more numbers derived from more and more “what if’s” plotting out bullet trajectories and wind drift amounts, and, always assuming a consistent bullet ballistic coefficient demonstration (also not likely) running “12” through all these mathematical-hypothetical scenarios will show that 12 doesn’t lose many, if any, points.

One last that isn’t really a strong point, but is a point… If we’re shooting something like a .223 Rem. then a half-grain is about 40 feet per second. If that 12 SD shows its worst and pops one out +36 feet per second, to me that represents something akin to a pressure spike (logic dictates that more velocity had something to do with more pressure). I know my loads are running a tad amount edgy, and seeing a small velocity variation is likewise a tad amount more reassuring that a primer won’t go over the edge.

tubb 1000 yard clean
Here’s the ultimate result of low velocity deviations. It’s up to the shooter to apply the left and right, but it’s up to the ammo to keep vertical stringing to a minimum. David Tubb does a stellar job on both. 1000 yards, fired prone with a scope. 6XC.

TESTING TIP
If you’re testing much beyond 200 yards, and especially beyond 300, pay no mind to the left and right, but keep a close watch on the up and down. In ideal conditions, groups are supposed to be round (I’m convinced they’re actually square, but there’s no need to go into that). If there’s any wind, don’t even try to correct for it (as long as impacts are on the target). I honestly don’t need a chronograph to confirm load consistency if I’m seeing small vertical dispersions. I’ll already have speed-checked the load I’m down on the mat with, and, again, I’m just wanting to see how level I get my perforations. If I come out with a 600-yard group that’s a foot wide but only three inches tall, I’m happy.

6 TIPS FOR LOWER SDs
Aside from finding the perfect and magical load combination, ha, there are a few things that do seem to help tighten shot-to-shot velocity deviations. They’ve all be talked all the way through and back again in this space in other articles, but, considered ultimately that this is the overall effect they have, here they are again:

One. Primer seating: fully seated onto a flat pocket bottom.

Two. Consistent propellant charge: weigh the charges if metering isn’t dead-on.

Three. Ignition efficiency: consider trying that inside flash hole deburring routine…

Four. Consistent case neck sizing, and, believe it or not, about 0.003 worth of “tension.” Don’t go too light…

Five. Temperature insensitivity: choose propellants that exhibit stability under extremes.

Six. Balance: strive to find a propellant that fills the case, but “loosely” (no compressed charges); even more, avoid an overage of air space. These both allow too much variance in ignition pattern.

inside deburring tool

This article is adapted from Glen’s books, Handloading For Competition and Top-Grade Ammo, available at Midsouth HERE. For more information about other books by Glen, visit ZedikerPublishing.com

Reloading Corner

RELOADERS CORNER: Standard Deviation

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Improving longer-range accuracy has a lot to with consistent bullet velocities. First comes understanding it! Here’s a start on it… KEEP READING

chronograph

Glen Zediker

It’s springtime (finally) and one of the things on your list might be working up a load for a new rifle, or new bullet. I’ve talked about testing processes and procedures, and also some about those bullets, and especially those with higher ballistic coefficients. The more aerodynamic bullet, by itself, is no guarantee of a smaller group (and whether you’re shooting one shot or 20 shots, you’re always shooting a group…).

To make the “magic” of a high-BC bullet come to life, they all need to be arriving at the destination at really close to the same speed. On target, that’s all about elevation consistency. It’s pretty commonly accepted among long-range competitive shooters that points losses come more from errant high and low impacts than from missed wind calls. High-BC bullets traveling at more consistent speeds reduces dispersions in all directions. But only if they’re traveling at consistent velocities!

The first step to improving velocity consistency is getting a good way to measure it. That there would be a chronograph. Nowadays especially, there are a number of simple-to-use and inexpensive chronographs available, that are accurate. Some have more features, which mostly revolve around providing printouts, digital records, and calculations, but what matters most (to me at least) is one that lets me easily read the velocity of each shot.

Check Misdouth offerings HERE

MagnetoSpeed
The newer barrel-mounted electro-magnetic chronographs make it really easy. I like the idea of being able to chronograph from shooting position, not just from a benchrest. This is a MagnetoSpeed.

So. What’s next is understanding the terms associated with this area of data-gathering.

“Standard deviation” (SD) is the most common measure of shot-to-shot consistency. It reflects on the SD reflects on the anticipated consistency of bullet velocities (some number of recorded velocities). The “standard” part reflects on a sort of an average of the rounds tested.

[Phrases like “sort of” upset mathematically-oriented folks, so here’s the actual definition: SD is the square root of the mean of the squares of the deviations. More in a bit.]

I pay less attention than many to standard deviation because: I don’t think standard deviation is near as important as is the “range,” which is the lowest and highest speeds recorded. Another that matters is “extreme spread,” which, by definition, is the difference between this shot and the next shot. I watch the speed on each shot. I compare this one to the next one and to the last one, and, as said, find the highest and the lowest.

Why? Well because that’s how I shoot tournament rounds. This one, then another, and another. A low velocity difference means that the accuracy of my judgment of my own wind call has some support.

standard deviation
Standard deviation calculation forms a bell curve. The steeper and narrower the apex of the bell, the narrower the fluctuations were. But there’s always a bell to a bell curve and the greatest deviations from desired standard are reflected in this portion of the plot. Depending on the number of shots that went into the SD calculation, these deviations may be more or less notable than the SD figure suggests. So? Watch each shot. That’s the way to know how a load performs with respect to velocity consistency. SD allows you to estimate how likely it is for “outliers” to show up.

A load that exhibits a low SD is not automatically going to group small, just because a low SD. I’ve had Benchrest competitors tell me that sometimes their best groups don’t come with a low-SD load, but do not apply that to greater distance! At 100 yards a bullet’s time of flight and speed loss are both so relatively small that even what some might call a big variation in bullet velocities (+/-25 fps or so) isn’t going to harm a group, not even the tiny groups it takes to be competitive in that sport. On downrange, though, it really starts to matter. (And keep in mind that “it” is a reference to velocity consistency, whether denoted by SD or otherwise.)

For an example from my notes: Sierra 190gr .308 MatchKing. Its 2600 fps muzzle velocity becomes 2450 at 100 yards and 1750 at 600 yards. (These numbers are rounded but serve for a example.)

If we’re working with a just awful 100 fps muzzle velocity change, that means one bullet goes out at 2550 and the next leaves at 2650, in the worst-case. The first drifts about 28 inches (let’s make it a constant full-value 10-mph wind to keep it simple) and the next slides 26 inches. But! Drop… That is THE factor, and here’s where inconsistent velocities really hurt. With this 190, drop amounts over a 100 fps range are about three times as great as drift amounts. This bullet at 2600 muzzle velocity hits 5-6 inches higher or lower for each 50 fps muzzle velocity difference. That’s going to cost on target, big time. And it gets way, way (way) worse at 1000 yards. Velocity-caused errors compound on top of “normal” group dispersion (which would be group size given perfect velocity consistency). Now, it’s unusual for a wind to be full-value and dead constant, so on-target left and right displacement is even relatively less — but elevation displacement is consistent regardless.

So, my 100 fps example is extreme, but half of that, or a quarter of that, still blows up a score, or an important hit on a target.

propellant charge consistency
This is probably the most influential factor in improving SD: consistent propellant charge. It’s not only that each case has an identical powder load, though, because primer factors, and finding the right combination ultimately is why we do all the testing…

So what’s a tolerable SD? 12. There have been, rest assured, much calculation to lead  up to that answer. That’s the SD that “doesn’t matter” to accuracy, meaning it’s not going to be the leading factor in a miss. It’s more than I’ll accept for a tournament load, but for those I’m looking for an extreme spread never more than 10 fps (the range might be higher, but now we’re just mincing terms). More later…

The information in this article is from Glen’s newest book, Top-Grade Ammo, available HERE at Midsouth. Also check HERE for more information about this and other publications from Zediker Publishing.

Reloading Corner

RELOADERS CORNER: SD Pt. 2

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Here’s how to use (or not use) Standard Deviation calculations in ammo decisions, what they are, and aren’t… Keep reading…

Glen Zediker

A standard deviation plotted out is a bell curve. Chances are outstanding that a range session calculation will plot into what they call a “normal curve.” Like any normal bell curve, it can get divided into three segments and given values, and, technically, these are the “standard deviations.” It’s “a” standard deviation rather than “the” standard deviation.

SD bell curve plot

Assuming a normal curve, the values are that about 68 percent of forecasted results will lie within one standard deviation of the mean, about 95 percent lie within two, and over 99 percent lie within three standard deviations. If we have an SD calculated to be 12, that means that applying one standard deviation means that about 68 percent of all “next shots” will be +/- 12 feet per second. Since, though, the curve is in threes, in effect if not in fact, that means that a scant number of the shots pose a chance for +/- 24 and some teeny chance remains for shots to go to +/- 36. That, however, is extrapolating or predicting with data and that’s not really wise and doubtlessly uncalled for. Data collection is a record of numbers and I do know that there’s 100 percent chance that the highest and lowest velocities collected for an SD calculation did, in fact, happen. That’s what matters. No matter what the shot results calculated into for an SD, those were the two that represent the highest and lowest prints on the target.

It’s mathematically not possible for an SD to be higher than the greatest single raw deviant, but I do for a fact know that an SD can easily be far lower than the worst shot. Given how it’s calculated, along with how many samples contributed, it’s plain that the nearer the majority are to themselves the less impact a bad one or more has.

I said in a very open-ended way last article that a tolerable SD is 12. Anything more than that is not good; anything less than that probably won’t perform noticeably any, if at all, better than a 12. However! It is at this number, so I say, where the often-uttered tune of “…SD doesn’t matter…” and its refrain of “…seen good accuracy with high SDs…” starts and stops. Twelve. That’s it. Now we have an SD that “doesn’t matter.” The reason this is stuck out here is that everyone has heard this chorus but hopefully figured that it couldn’t be taken universally at literal value. Well it can’t.  So now you know! It’s 12. 12 should not be responsible for a points loss, even accounting for or including coincidence of any one shot hitting the edge limit of usual group size.

(Yes, 13 or 14 or 16 or even 20, which is often given as a “limit,” might well be a realistic ceiling but I drew a line to have one. Since there’s a line, now we can cross it and commence argument. I won’t use any load in competition that wouldn’t calculate to a single-digit SD. My 600-yard .223 Rem. load tested to an SD of 3.18 with a Range of 8 fps.)

So after all this has been said, I don’t give SD as much weight in my load decisions as some do. The reason for my focus on it here, as said in the first article, is because that’s the usual “standard” measure of consistency. I look at the speeds as they come up on the chronograph display and write them down. I weigh range and extreme spread more heavily, and I want to see really small variations over the number of test rounds I fire. It’s a matter of waning patience and waxing time. If I see a variance that could cost a point, that load is abandoned.

TESTING TIP
If you don’t have a chronograph or don’t want to burden a testing session with using one, watch for a correlation between the elevation dispersion and the wind dispersion of test groups. At 600 yards I always test from position (prone, “suited up”). No chronograph (muzzle-mounted chronographs now make this a non-issue). I’ll already have speed-checked the load I’m now down on the mat with. When I shoot my groups, I honestly don’t pay much attention at all to anything but measuring how level I got my perforations. Attempting shot-to-shot wind corrections when testing for ammunition accuracy throws another variable into it that might be most misleading. If I come up with a group that’s a foot wide but only three inches tall, I’m happy.

5 TIPS FOR LOWER SDs
Aside from finding the perfect and magical load combination, ha, there are a few things that do seem to help tighten shot-to-shot velocity deviations. They’ve all be talked all the way through and back again in this space in other articles, but, considered ultimately that this is the overall effect they have, here they are again:

One. Primer seating: fully seated onto a flat pocket bottom.

Two. Consistent propellant charge: weigh the charges if metering isn’t dead-on perfect.

Three. Ignition efficiency: consider that inside flash hole deburring routine…

Four. Temperature insensitivity: choose propellants that exhibit stability under extremes.

Five. Balance: strive to find a propellant that fills the case, but “loosely” (no compressed charges); even more, avoid an overage of air space. These both allow too much variance in ignition pattern.

Propellant level.

Addition
I learn things all the time. A most knowledgeable and helpful reader pointed out a detail in SD calculation that is better adapted to calculations for ballistics, and it helps because of the usually relatively small size sample involved. We’re not going to chronograph 100s of rounds, usually 10-20. So, instead of dividing the average square of the deviations by the number of samples, but the average square of the values, less one (n-1). That helps any distortion of results toward a number that calculates too small. Keep in mind always that SD is an estimate, in one way of looking at it.

This article is adapted from Glen’s book, Handloading For Competition, available at Midsouth HERE. For more information on that and other books by Glen, visit ZedikerPublishing.com

Reloading Corner

RELOADERS CORNER: SD — what it matters and why

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Getting a handle on improving long-range accuracy has a lot of to do with understanding the importance of consistent bullet velocities. Here’s a start toward that…

Glen Zediker

There’s one more thing (seemingly, there’s always one more thing…) that’s important to accuracy at extended distance. I’ll say “extended distance” is anything over 300 yards. That is bullet velocity consistency.

I’ve said in these pages before that a good shooter will lose more points to elevation than to wind. This next explains that a might more.

First, and the first step, is getting and using a chronograph. It doesn’t have to be a zoot-capri model, and nowadays that’s a fortunate bonus because there are a number of inexpensive chronographs available that are entirely accurate.

PACT chronograph.
A chronograph is essential, well, at least for all this here. There are a lot of good ones. I’m partial to PACT.

Check Misdouth offerings HERE

magnetospeed chronograph
The newer barrel-mounted electro-magnetic chronographs make it really easy. I like the idea of being able to chronograph from shooting position, not just from a benchrest. This is a MagnetoSpeed.

“Standard deviation” (SD) is probably the most commonly used measure of bullet velocity consistency. SD reflects on the consistency of velocity readings taken over a number of shots. “Standard” reflects on a sort of an average of the rounds tested.

[Math folks don’t use phrases like “sort of” when describing numbers and can provide tickier definitions of SD and the means to calculate it. Here it is: it’s the square root of the mean of the squares of the deviations. Actually harder to say than it is to calculate.]

Steady Wins the Race
Standard deviation is not the only measure, and I don’t even think it’s the right one, let alone the most important, but it’s no doubt the most popular way to talk about ballistically consistent bullet performance. I don’t think standard deviation is near as important as is the “range,” which is the lowest and highest speeds recorded. Some who write and talk about it call that “extreme spread,” but if we want to get picky over terms (and ballisticians, card-carrying and self-styled, tend to get right touchy over such formalities) extreme spread is the difference between this shot and the next shot.

I watch the speed on every shot. I compare this one to the next one and to the last one, and, as said, find the highest and the lowest.

There is no saying that a load that exhibits low standard deviation is going to group small, just because of that. Any Benchrest competitor will tell of experiences whereby “screamer” groups came with high SDs and hideous groups with low SDs (“high” and “hideous” by their standards, still pretty small for the most of us). But, at 100 yards the bullet’s time of flight and speed loss are both so relatively small that variation in bullet velocities isn’t going to harm a group, and, yes, not even the tiny groups it takes to be competitive in that sport. On downrange, though, there is going to be a relatively greater effect on shot placement, right? Yes and no. Drift and drop are influenced. There is a relatively greater effect in ultimate displacement of elevation, more next. Based on drift allowance it probably does not.

To put an example on it, let’s say we’re shooting a Sierra® 190gr .308 MatchKing. Its 2600 fps muzzle velocity becomes 2450 at 100 yards and 1750 at 600 yards. (All these numbers are rounded examples, and examples only.)

If we’re working with a truly hideous inconsistency of 100 fps, say, that means one bullet goes out at 2550 and the next leaves at 2650 in a worst-case event. The first bullet tracks across about 28 inches (constant full-value 10-mph wind to keep it simple) and the next moves sideways 26 inches. Figuring drift on 2600 fps means it’s two inches off, one inch per shot.

Drop, which means elevation, is a (the) factor, and here’s where poor SDs bite. With this Sierra® 190, drop amounts over a 100 fps range are about three times as great as drift amounts. A vertically-centered bullet at 2600 fps hits about 5-6 inches higher or lower at each 50 fps muzzle velocity difference. That’s enough to blow up a score to elevation. And it gets way, way on worse at 1000. Keep always in mind that velocity-induced errors are compounding “normal” group dispersion. And, in reality and as discussed before, it’s unusual in a competitive shooting venue for a wind to be full-value, so the on-target lateral displacement is even relatively less — but the elevation displacement is consistent.

The next-to-the-bottom line, then, is that poor SDs don’t hurt in the wind as much as they do on the elevation. The bottom-line, then, is back to the start: don’t shoot a load with inconsistent speeds. It’s flat not (ever) necessary.

So what’s a tolerable SD? 12. That’s the SD that “doesn’t matter” to accuracy. More later…

SD bell curve
Standard deviation calculation forms a bell curve. The steeper and narrower the apex of the bell, the more level and narrower the fluctuations were. But there’s always a bell to a bell curve and the greatest deviations from desired standard are reflected in this portion of the plot. Depending on the number of shots that went into the SD calculation, these deviations may be more or less notable than the SD figure suggests. So? Watch each shot. That’s the way to know how a load performs with respect to velocity consistency. Each and every speed collected for each and every shot fired in a test. More next time…

MATH: For Them That Wants It
If you have no electronic gadgetry to help, calculate SD like so: add all the velocities recorded together and divide them by however many there were to get a mean. Subtract that mean number from each single velocity recorded to get a deviation from the mean. Square each of those (eliminates the negative numbers that ultimately would cancel out and return a “0”). Add the squares together and find the mean of the squares by dividing again by the number of numbers. Then find the square root of that and that’s the standard deviation figure, which is “a” standard deviation, by the way.

This article is adapted from Glen’s book, Handloading For Competition, available at Midsouth HERE. For more information on that and other books by Glen, visit ZedikerPublishing.com