The following is a specially-adapted excerpt from the book Handloading For Competition, by Glen Zediker. Visit BuyZedikerBooks.com for more.
by Glen Zediker
A “ballistic coefficient,” or “BC,” is a number that reflects on the aerodynamic performance of a bullet, how well it flies.
I was explaining BC to a fellow once and after talking through all the technical language he said, “So, it’ll hit furtherer on up the hill….” Exactly.
It’s actually a comparison, and that gets explained first. Here’s how it
works: There are “standard” bullets that are mathematical models. Workaday ballisticians know which model to apply to different bullet styles. For most rifle bullets we’ll encounter, one model is a “G1” (there are other models, like G7). The flight of this bullet has been calculated at varying velocities and distances. Pistol bullets, for example, are calculated from or compared to different standard bullet models.
The standard bullet, and, again, let’s say that’s a G1, has a BC of 1.000. An actual bullet that’s compared to the G1 at points, distances downrange, will either be flying faster or slower than the G1. If it’s faster, its BC will be 1.000+; if it’s slower, it will be 1.000- (fractional).
Comparing bullets with different BCs, the one with the higher number will lose less speed over distance. Losing less speed means its flight time will be shorter and it won’t drift and drop as much as a bullet with a lower BC. So, a 0.600 flies better than a 0.550. So: the higher the BC, the less speed lost over distance. That’s it.
Published or stated BCs are either calculated or measured, depending on the maker’s policy. More mathematics than I can wrap my mind around can get these calculations done based on a bullet’s blueprint. Measured BCs involve chronographing at the muzzle and then at other points on downrange, same bullet, same flight. There’s a good question as to which provides the best information. Some logic applied suggests that, without question, a measured BC is more “real world” and therefore more valuable. On the other hand, if the point is to compare bullets, then calculated BCs might be more reliable. One point, however, is that the relationship between measured BCs and calculated BCs is that measured are usually lower…but not always. Reasons for that follow.
All the drift and drop tables (whether printed or digital) you’ll see are based on a bullet’s BC. And, the accuracy of those tables clearly revolves around what the actual BC performance of the bullet you’re shooting is.
So what affects the actual, realized BC of a bullet? A lot of things… Anything that can influence bullet flight influences BC realization. Bullet stability has the lead, though. For a supposed BC to be realized, the bullet has to be “asleep.” If it’s not stable, it’s encountering disruptions that will slow it down. I don’t know many who have had much luck running BC tests “at home.” That’s a logistics issue with chronographs, as could be imagined. Those, however, who have successfully done their own BC testing learn a lot. One, for instance, is that even the rotational speed of a bullet in a test can influence BC. Comparing the same bullet through a 1-8 and a 1-7.5 twist barrel, the 1-8 likely will net a higher BC. The extra revs per second from the faster twist are the likely cause. Easy enough to imagine: 1000-yard BC tests are more revealing than are 500-yard tests.
Atmospherics, which can be a long list of factors, influence BC mightily. Air density is probably the most powerful influence here. Any conditions that allow for easier passage of a bullet through the air don’t detract as much from its BC as any conditions that do serve to impede its flight. A BC, which is based on sea-level air density, can easily show itself as a higher number at 2000 feet above sea level.
Reality is that the demonstrated BC changes from morning to afternoon and day to day and place to place. The calculated BC is not changing, of course, but the mistake is assuming that a BC is some “set” or finite measure of bullet performance. If you’re interested, there’s some valuable information from David Tubb (visit DavidTubb.com). He’s done a volume of work on calculating influences from atmospherics as it applies to his DTR project, which, in one way of seeing it, gets down to understanding why it’s really rare to dial in what a ballistics table says for a particular bullet and speed and distance, and hit the target.
One last bit of information I’ve always found interesting is that a BC is a finite thing, whether the bullet at hand is going to show it or not. Any BC derived from a G1 model, for instance, fits all bullets with that same BC. This was helpful before ballistics apps were as common and easy as they are now. For instance, if there was a new .224-caliber bullet with an advertised BC, but no tables, just find another bullet, of any caliber, with that same BC, plug in the velocity, and the drift and drop figures would be accurate. It doesn’t matter if the other bullet was a .308 or .277 or whatever else.
“….a 1-8 and a 1-7.5 twist barrel, the 1-8 likely will net a higher BC. The extra revs per second from the faster twist are the likely cause. ”
Somebody failed in the “proofreading” department! Quit reading after that.
No, it may have been written unclearly, but you’re misreading the point: over-stabilization from a too fast twist can result in a lower maximum effective range. He is saying the faster 1-7.5″ twist is the cause of the lower BC.
You are incorrectly assuming that he meant that the 1-8 twist is a faster twist rate.
Again, it could have been written better, but in any case you quit reading too soon…
Correct!
“….written unclearly….” We can definitely agree on that; however, as written there is no ‘assuming.’ As stated, “Comparing the same bullet through a 1-8 and a 1-7.5 twist barrel, the 1-8 likely will net a higher BC.” Even considering the following sentence, the ‘whole’ is incomplete and incorrect. The BC will ‘primarily’ depend on that “same bullet(‘s)” length and weight, in relation to the twist rate of the rifle! The “assuming” has to begin, in order to make any sense of those two sentences!
You also are incorrect in “your assumption” that he is talking about “over stabilization” influencing BC. He does not even mention “over stabilization.” There is only too much rotational speed, or too little – not “over-stabiliztion,” i.e., the bullet is either stable or unstable. I have witnessed bullets blowing up before hitting anything, by having their ‘rotational speed’ parameters exceeded, but not from being “over-stabilized.”
He stated no bullet weight/length. Heavier/longer bullets, in caliber, more “stable” w/faster twist rate, lighter/shorter bullets, in caliber, more “stable” w/slower twist rate. Part of the reason, in .223/5.56 caliber, the heavier (70-85 grain) bullets are used w/mostly 1:7 twist barrels for long range competitions.
Finally, “….a specially-adapted excerpt from the book Handloading For Competition….” I would hope that the “excerpt” attempt is the failure here, and not what was actually written in Mr. Zediker’s book.
You know, I agree with most of your points, but was responding to your initial post where you misunderstood what he wrote. Write all you like now to explain the flaws in his statement, all of which I can see, but that’s not the original point and counterpoint.
And the term “over stabilization” is a normal synonym for a too fast twist rate, which is what the author was trying to say causes a lower BC in this specific example. I think you’re parsing it now because you now see my point and what he was saying. Yes, he didn’t point out all the bullet variables that could negate his statement, but again, that’s not what we were talking about. You assumed because of the poor sentence structure that he was saying 1-8 is faster than 1-7.5, that’s the discussion here, not your new additional points.
I have AR’s with 1:9, 1:8, and 1:7 twist rates and am familiar with the differing performance of different weight bullets in those various barrels. I also have a Savage bolt-action 1:9 twist that loves 70 grain bullets, while the 1:9 AR does poorly with them, and can only attribute that to the tighter chamber and longer barrel.
Finally, I think the author is knowledgeable, but don’t think this excerpt with its construction problem is an exception unfortunately. Lots of books being cranked out these days, and many suffer from a lack of careful proofing.
I try to keep the word counts to a minimum for these online articles. It seemed pretty clear to me that given a 1-8 and a 1-7.5 folks would identify which was the faster twist… And so, the faster twist can mean a lower demonstrated BC. I do see it could have been spelled out better. My mistake.
I also caught that error
This was shown clearly by David Tubb’s testing on a 115 RBT using an Oehler 88 system, same bullet different twist 6mms.
I suppose I’m just plain stupid but I got the drift of what the writer meant. I had to go back and figure what your problem was in the interpretation of the meaning. I would also assume that if a bullet could stay stable without twist it may well have a higher BC because it would come into contact with less air but that comes from a redneck that just wants to shoot f”a”rther up the hill.
I don’t know what the argument is, as a generalization he is correct; spinning a bullet too fast will affect BC. If we want to argue specifics, say a 77 Grain 5.56 bullet out of a 14.5″ barrel then you would most definitely want the 1:7 barrel vs 1:8, and even that is not a given since I have a 1:9 that likes them.
Mr. Zediker, thanks for confirming and clarifying. FYI, did not realize you were the author of a book on correctly using service rifle slings. I have a Garand and a couple of other rifles set up with the Brownell’s Competitor Plus slings, and it’s always a challenge to get them adjusted 100% correctly. I’ll look to pick that up as well as your book on competition AR mods.