Reloaders Corner: Setting Cartridge Case Headspace

The following is a specially-adapted excerpt from the forthcoming book, “Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing. Click here to order.

by Glen Zediker

A rifle chamber has a headspace; a cartridge case has a headspace. The first is set by the chamber reamer and its operator; the next is up to us on the loading bench.

This is the loose working definition for headspace necessary to understand for this next: It’s the space from the bolt face to the “datum line” on the case shoulder. SAAMI sets standards for this dimension for each cartridge, and a gunsmith or manufacturer can have a little leeway in establishing the exact dimension in a rifle chamber.

In practical terms: Chamber headspace dimensions are fixed; cartridge headspace dimensions are variable. In a semi-auto, there should be some room, space, ahead of the case shoulder when the round is fully chambered, not be a perfect, flush fit. This is necessary for safe function in a semi-auto. In a bolt-action, the difference between chamber headspace and cartridge headspace can be miniscule to non-existent.

A little extra space helps ensure reliable functioning in a semi-auto, and also, mostly, precludes the chance that the case might bottom out on the shoulder area in the chamber before the bolt is fully locked down.

Back to more definitions. A datum line is actually a diameter; the line itself (the point set by the headspace dimension) is the point along a case shoulder that coincides with this diameter. There are only 5 datums that apply to virtually all bottleneck rifle cartridges. (Modern bottleneck rifle cartridges headspace off the case shoulder. Belted magnums and rimmed cartridges are different stories, for a different story.)

To correctly resize cases, we need a gage. Of course we do! I prefer the one shown in this article. It reads from case base to datum line, just as it’s done in chambering a barrel.

I talked before about how semi-autos can often exhibit case shoulder growth in measurable excess of the chamber. Meaning: the case shoulder can be free to expand beyond the confines of rifle chamber dimensions, and that is from the premature bolt unlocking that accompanies most every gas-operated rifle. Even when the system is working optimally, the case shoulder can advance slightly as the bolt just begins to unlock and move away because the case is still containing pressure. The severity of the discrepancy has mostly to do with how much the gas system is overloaded.

To refurbish a spent case, the case body outside needs to be shaped up to near-to-new dimensions, and also the case shoulder needs to be “set back” an adequate amount to ensure positive chambering with just a tad of “rattle” between the chamber shoulder and the case shoulder. In a bolt-gun, the case emerges from the chamber holding essentially a dimensional mirror of the chamber itself (minus, always, the 0.001 “spring back” inherent in the brass material).

For best results, and case life, we need to figure out how much to set the shoulder back. Too much really won’t affect load performance, but, in my belief, deliberately creating what amounts to excessive headspace is not wise. It’s just that much more expansion, that much more “working” the brass has to endure. However! That’s not nearly as bad as leaving the shoulder too high! That’s dangerous.

For day-in, day-out use, I suggest setting back the case shoulder 0.004 inches from fired case dimensions for semi-auto ammunition. If you keep the chamber clean, then go with 0.003. I think that 0.002 is not enough in a semi-auto, but that is plenty enough in a bolt-gun, and 0.001 is the minimum. Commence argument, but I’ll stick by those numbers.

The numbers we need to get from our gage are these: new, unfired case shoulder height (where we started); fired, unsized case shoulder height (where we went to); sized case shoulder height (where we need to get back to).




27 thoughts on “Reloaders Corner: Setting Cartridge Case Headspace”

    1. If your bolt closes easily then everything is just fine. If the case shoulder is “out” too much that’s often what causes the hard bolt closing on a bolt-gun. In a semi-auto there are no such “feels” involved in cycling. And, yes, if the bolt doesn’t close easily on a semi-auto it is dangerous.

      1. I am sure you meant “closes eailsy” after having found the proper headspace. It is possible that there are some out there new to reloading that would not understand that a drastically undersized case with excessive headspace would definitely let the bolt close easily. It would also allow the case head to eventually separate easily. This was something I hoped he would have mention in his article. Instead he just mentions that the brass works more than needed. This working more than needed is allowing the case head to get thinner and thinner just begging for a rupture. You cant go too much and going too little is dangerous. He mentions danger in too much but really, too much will disengage (on a semi Auto and on a bolt it wont close so it wont fire) the sear and not allow the gun to fire. Too little headspace and the gun could fire (misfires are a symptom of too much) and if it fores then you are i danger. Quite the opposite of what he stated.

      2. If you size down a case, a little at a time, for a bolt gun, when the bolt closes over (assuming there’s been some resistance along the process of adjusting the sizing die) then that should be enough shoulder set back, and also not too much. Following the suggestions I offered to use a gage to compare fired case headspace against sized case headspace, nothing will be excessive. If someone wants to crunch down a case shoulder as far as a die might allow then, yes, that could be excessive, especially in a rifle with an oversized (in any dimension) chamber. But that’s not going to happen if someone follows the process outlined. 0.004 inches of shoulder set back is in no way excessive, not for a semi-auto that’s not going to get dozens of firings from a case. If the shoulder is left “out” too much, that can create big problems, and one is from the floating firing pin on AR15s, M1As, M1s, and others. The firing pin “taps” or bounces into and then off the primer when a round is chambered. Any resistance to bolt closing can create a “slam-fire” and I’ve seen enough of those to warrant that nobody wants one. The main culprits in that condition are overly sensitive primers and primers that are seated too high, which is partially replicated by feeding an insufficiently-sized cartridge into a semi-auto chamber. The closing force of a cycling action is enough to set back a high shoulder, but the resultant lag in progress is where the danger is.

    2. I don’t get that from his article. If you neck size then the shoulder and body stay in the fired dimensions. he mentioned this is .001″ from the mirror dimension of the chamber. He goes on to state that .001″ is the minimum. So that means he recognizes that .001 and/or neck sizing is reasonable. The .002″ (2 as in too much for me) was his preference but I never got the impression that neck sizing was dangerous, just not his cup of tea. To be clear, I neck size, ,I also anneal fairly often to make sure the spring back he talks about stays consistent in my neck/shoulder area. I guess it comes down to whether or not you want accuracy (start the argument) or a safety margin for consistent firearm function.

      1. That’s exactly it. And I respectfully do have argue a tad amount about accuracy comparisons. I’ve found that case and component segregation, and case prep, along with (correct) use of good tooling, matters the most. Benchrest competitor’s loading tactics (which is a source for a lot of what we generally regard as “accuracy” loading techniques) are based or built around their essentially unified construction between case and chamber. Most winning shooters HAVE to turn case necks to suit chambering specs, and it’s common for them not to size necks even a little bit shot to shot, and never trim cases (they tend to shorten firing to firing). Most applications just can’t replicate that, and it will not happen in a factory rifle using out-of-the-box cases. Any rifle with an ejector is going to warp cases, for instance. Many of us see improved concentricity from body sizing. As with most things, the way we all get our methods is through experience. I appreciate that many have found they get the best results through neck-only sizing; however, also appreciate that others find it doesn’t help. I think that depends on the quality of rifle construction, tooling setup, chamber dimensions, and a list of other factors probably not all fully predictable. David Tubb’s current NRA High Power Long Range record of 1600/1600 was fired with full-length sized cases. Many of the top shooters prefer using new (carefully selected and prepped) brass for an important event. “We” get pretty good groups from that process. I’m not arguing even a little bit against neck-sizing for a bolt-gun, as long as the operator is fully cognizant of fully understanding the process (and mostly, for my sake in writing these articles, that there’s an understanding of why it shouldn’t be done on a whim, and never for a semi-auto, functioning as a semi-auto at least).

  1. I understand what is being said in the article about head spacing. However, a little about how it works and cost would help me make a decision in purchasing the tool.

  2. Mr. Zediker
    Your article was excellent, especially the information that a “fire formed” case from a semi-auto is not a true chamber dimension as well as the reason why (premature bolt unlocking) and the numerical value (0.001) for case “spring back”.
    Question #1
    In regard to setting back the case shoulder 0.004 inches from fired case dimensions for semi-auto ammunition.
    Because of this small set back amount, would this value be subtracted from the lowest, or average, or highest measured fired case dimension from a sample batch. Also what number of cases would you suggest to make this sample batch?
    Question #2
    For rifles that have a gas port shut off valve like the M14/M1A and the FN FAL, when closed, would the “fire formed” case minus “spring back” (0.001) represent a true chamber dimension measurement? If not why?
    Question #3
    Is the premature bolt unlocking phenomena the cause of slightly bent case rims and can this be reduced or eliminated by using a lower propellent charge?

    I already have your books “Handloading for Competion” and “The Competitive AR15: The Ultimate Technical Guide” and shall look forward to your forthcoming book “Top-Grade Ammo”. When is the release date?
    Thank you.

    1. It’s always a good idea to measure a few cases. Always err on the safe side if there are any differences measured. And, yes, if the gas system is shut off, then the fired case dimensions will pretty much reflect on the chamber itself, like they do in a bolt gun. And, also, yes, the bent case rims are from the extractor yanking that hard on the case rim. Too much pressure (gas port pressure). Thanks for the questions!

  3. This is good knowledge for someone who is getting in to comp shooting or getting unknown brass. Head-spaceing is not a waste of time and money and it is easy to learn and do. That being said I haven’t done it but a cupple of times this month for other people, and has been over a year on anything I have. 4 days ago at LBL Public gun range ($0 free) I took one piece of 308 brass, a can of Varget, 100 BT Match Palma 175gr, 200 (I tend to drop a few) Winchester large rifle primers and a Lee loader with a small mallet and shot ten groups of ten at 100 yards with a $300 Savage rifle and a $400 SWFA 10X40 SS scope on it. It only takes about 45 seconds per round to reload, average about a round a minute, largest group was my first at 1.18 the remaining 9 groups were all under .61
    I put 500+ rounds a week down range, reloading is a must but it doesn’t have to cost a fortune and you don’t need a ton of equipment to do it.
    PS. Takes too much time. Cheaper to go get new brass. Lol

    1. I agree! I just replace my brass on a routine instead of taking the hours and hours (over enough cases) to fix it back for one or two more uses. I like to keep it consistent. More changes mean more adjustments to tooling also. I remember asking Middleton Tomkins (one of the King Gods of long range rifle shooting) about case annealing many many years ago. Here’s my best recollection of his response: “When my cases need to annealed, I fill a pan with water to reach halfway up the case body. I then carefully stand each case up in the pan. Then I tip it over into the trash can…” Haha. But my feelings exactly.

  4. Lately I have been reading a lot about how a gauge for measuring cartridge headspace is an intregal tool for a reloader of rifle cartridges. The authors cite the danger of pushing the case shoulder too far back during resizing, thereby creating excessive headspace and possibly leading to case head separation.
    I have been reloading for over forty years and I have never used a cartridge gauge. Neither have I had a problem with the many thousands of rifle cases I have reloaded and fired during that time. The only problem I have encountered was due to excessive headspace of the rifle chamber.
    One of the most common mistakes I have seen with other reloaders is their failure to use the proper shell holders when resizing. SHELLHOLDERS FROM THE VARIOUS DIE MANUFACTURERS ARE NOT INTERCHANGEABLE, not even those for the same specified cartridge. (Using the wrong shellholder can cause the case to be pushed up too far into the sizing die.) And, if there is any question as to whether a rifle has the proper chamber dimensions and headspace, such as surplus military rifles with mismatched bolts, then the rifle’s headspace must be checked before that rifle is considered safe to fire. Cartridge headspace gauges will not help with this.
    So, my opinion in this matter is to save your money for cartridge gauges. As long as you are reloading ammo to be fired in a Ruger, Remington, Winchester, Savage, etc. firearm of recent manufacture or have confirmed the firearm has proper chamber dimensions, AND are using the proper shell holder matched to the sizing die of the same manufacture, you will never need a cartridge gauge.

    1. A gage gives some numbers to work with. I disagree with the comments on shellholders. It shouldn’t be necessary to make full contact with the die bottom, and, again, that’s where gages come in. I tend to use Lee-brand shellholders for all my presses. They are (surprisingly) very correct and consistent; there are almost always some variations in shellholders.

      1. When you stated that the shell holder does not need to make full contact with the bottom of the sizing die you are describing neck sizing. And, when neck sizing the shell holder brand is not critical, although I still think that using the same brand shell holder on all your presses (I assume you mean with all brands of sizing dies) is an extremely poor practice. Neck sizing, under certain conditions, is a great idea. It extends case life by reducing metal fatigue and probably improves accuracy by tailoring the case more closely to the exact dimensions of the rifle’s chamber. The problem is that a neck sized case must be fired in the same gun as the one that it was last fired in. If you try to chamber a neck sized round in a different gun of the same caliber it may fail to do so, or it may chamber only with difficulty and result in increased chamber pressures. I have five 8mm Mausers, four .308s, three 7mm Mausers, three 7.65 .53s, and two .270 Wins. I fire all these guns and I don’t keep track of which gun fired which cases so I usually full-length resize.
        When reloading I make it a practice to adhere to some very strict rules. These ruled incorporate some words that convey absolutes, such as “always” and “never” and “must”. I adhere to these rules even when doing so may not seem necessary. By following these rules I have so far been able to keep both eyes, all of my teeth, and both hands and all of my fingers. I know several people who have not been so lucky. And so in the future when I’m resizing ANY metallic cartridge I will ALWAYS use the proper shell holder for the cartridge I am working with and of the same manufacturer of the sizing die I’m using.
        Did I mention that I’ve been reloading for over forty years?

      2. Properly adjusting the sizing die into the press is probably more important than which brand shell holder you use, because this is how you adjust your cartridge headspace. The arbitrary set up of 1/8 to 1/4 turn of the die after contacting the shell holder CAN get you in trouble. Just as rifle chambers vary to specifications, DIES do as well. So unless you are measuring, you may be resizing your brass too short.

      3. The fact that you are downplaying the importance of using the proper brand shell holder indicates to me that you are probably using the wrong shell holders when you reload. And, you are quite correct that that reloading die dimensions vary. THAT, is one reason why you need to match the brand of shell holder to the brand of resizing die.
        It has been my experience that if you are using the proper sizing die/shell holder combination it is virtually impossible to push the shoulder of the case back far enough to create an excessive headspace condition.
        There are two possible exceptions. One would be if one or both of the items are defective or mis-labeled. The second would be if you are loading 5.56 or 7.62X51 ammo using .223 Rem. or .308 Win. dies. If so, OK then, measuring for proper dimensions of sized brass could be useful. But, why not just get 5.56 or 7.62X51 dies?
        In my opinion the existence of “cartridge headspace gauges” is what CONSUMER REPORTS might refer to as a “pack” which is an unnecessary item of merchandise or a service that a salesman or marketer tries to convince the buyer that they can’t live without, and that has the potential for a high profit for the seller.
        But, if it will make you feel any better go ahead and lay down your hard-earneds for “cartridge headspace gauges” instead of paying a couple of bucks for the correct shell holders. And, the next time you buy a new TV set make sure to get the “extended warranty”. Oh and when you order your new car from the Chevy dealership in Brainerd make sure to tell Jerry Lundegarde that you want the “Tru-Cote”

    1. If there is too much space available in the chamber (the case fits too “loose”), then the cartridge case will fill it when it’s fired. Doing so creates that much more stretching, and stretching creates weakened areas. The stretching is toward the rear or base of the case, in the “case head” area.

    2. Excess headspace is a condition which usually occurs with firearms which fire bottleneck rimless cartridges and involves the size of the rifle’s chamber when too much space between the bolt face and the chamber’s inner shoulder exists. The condition can be caused by excessive wear to the bolt’s locking lugs, a mismatched bolt, or by a mis-installed or poorly chambered barrel. Excessive headspace can also occur when the rifle’s chamber is correct but when the cartridge’s shoulder has been pushed back too far because the wrong shell holder has been used during full length resizing.
      When a cartridge is fired in an excessive headspace condition here is what happens: At the strike of the firing pin the cartridge moves forward in the chamber until the shoulder contacts the inner shoulder of the chamber. When the primer and powder ignite the internal pressure causes only the forward section of the case where the brass is thinnest to expand and adhere to the rifle’s chamber walls. The internal pressure then causes the rear of the case where the brass is thickest to move rearward and slam into the bolt face. This causes a stretching of the brass forward of the case head and creates a weak point. This weak point can be usually identified by the existence of a bright ring about 1/4 to 1/3 of the way forward of the case head. At this point the brass is compromised and should not be re-used. To do so could cause case head separation, which can also occur during the initial firing. Case head separation occurs when the integrity of the case is breached during firing and hot gasses and possibly debris is released into the rifle’s chamber and back through the action in the direction of the shooter. Some guns, such as Mausers are ported to channel escaping gasses away from the shooter, and some are not. In any event, case head separation can result in damage to the firearm and injury to the shooter and it, as well as excessive headspace which is the root cause, is to be avoided.

  5. Great article on head spacing, my wife laughed her butt off on your annealing knowledge!
    Now I will say (painfully true) her and my daughter are more accurate than me when it comes to groups at any range, with anyrifle, from 204 to her 20/50 Anzio. I find the name of your book Top Grade Ammo to be the point and my wife with her little Lee loader and Savage rifle with 2 1/2 hours of chamber & bore polishing – recut crown and glass bedding to be a big block in a Vega.
    The same rifle (Samantha) with “Top Grade Ammo” shoots sub .25 inch groups which she failed to point out.
    She is correct in stating you don’t need a lot of equipment or a lot of money to do it, but once you have the basics and you think you got your bases covered and you show up at the range with a $5000.00 rifle and scope with sub grade ammo and a grayhaired woman with a (P.O.S. looking) Savage and a little Lee loader takes your money or worse (she has came home with 2 AR’s and a 30/378 Weatherby) your book Top Grade Ammo might make the list on shit you may need to know! Just saying.

  6. I was admiring the neck sizing guage on the end of your Starrett calipers. Can you provide the name of who manufactures it and where one could locate such a nice guage?

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