RELOADERS CORNER: Cartridge Case Headspace

Knowing, and controlling, this dimension is a crucially important step in the case sizing operation, especially for semi-autos. Here’s what it is and why it matters. Read all about it!

Glen Zediker

Last time, and to start the new year off, I hit a few highlights on the first of what I think are some of the most important things to understand in reloading for bolt-action and semi-automatic rifles. A majority of those differences is in what’s allowable and possible in cartridge case sizing.

The reason I’m running these articles is to clearly define the differences in, essentially, what you can get away with (and can’t get away without) depending on the action type. Don’t confuse some of the tactics, tools, and techniques used for bolt-actions and (mis)apply them to semis. That can range from frustrating (function issues) to disastrous (blowed-up guns). I hope that these focused articles will clarify the basics before moving on to the finer points respecting each.

case headspace illustration
Here’s headspace: it’s a height based on a diameter. A .223 Rem. uses a 0.330-inch-diameter datum; the height to the diameter on the case shoulder that equals 0.330 inches is the headspace dimension, measured from the case base (this is measured from the bolt face to determine headspace in a rifle chamber). There are only 5 datums that apply to all standard bottleneck cartridges; the correct number for your cartridge will be referenced in the cartridge specifications. (Belted magnums and rimmed cartridges are different stories, for a different story.)

Following on that, here’s one: cartridge case headspace. A rifle chamber has a headspace; a cartridge case has a headspace. The second cannot exceed the first. Here’s how it goes:

The area in point is the case shoulder, the area between the bottom of the case neck cylinder and the case body. There are two dimensions associated with case headspace: the diameter of the “datum” line, and the height (measured from the case base) to that line. So, headspace is determined by the location of the datum line. There are only 5 datum diameters in use over the range of bottleneck rifle cartridges. Datum diameter will be indicated in the cartridge description in any good loading manual. (Belted magnums, which headspace off the belt, are the exception, and different stories, and so are rimmed cases.)

Chamber headspace is determined by the chamber reamer and also the one operating the reamer. There are SAAMI standards for all standard cartridges (which are coincidentally those having SAAMI specs). Ammo manufacturers set their cartridge case dimensions to work within those same specs, and almost always with (literally) some room for variations. That means that, usually (and, again, I’m talking about factory-chambered rifles) the cartridge case headspace will be a little shorter than the rifle chamber will accommodate.

When a round fires, as is by now well-known, the case expands in all directions under pressure, swelling and conforming to the chamber, then retracts immediately afterward when pressure dissipates. Since brass has a plastic property, dimensions are not going to return to exactly what they were prior to firing, and that’s what all the sizing tools and operations seek to rectify. So, among other changes, the case shoulder will have “blown forward,” after having snugged up into that area of the rifle chamber. That will have moved the datum line upward. As hit upon last article, semi-automatics are notorious for exhibiting a little more than they “should have” in expanding, and that’s because there’s a little (to a lot) of pressure latent in the case when the bolt starts to unlock and move rearward. This can effectively create additional space for case expansion within the chamber. The case shoulder measurement after firing in a semi-auto might actually exceed that of the actual chamber headspace, or, at the least, be a little taller than it would have been in a bolt-gun having the exact same chamber dimensions. The hotter the load, the more gas system pressure, the more this might show.

case headspace tools
Get a few de-primed once-fired cases and a gage and get to work. Here’s a Forster Datum Dial gage. Works well and works for all standard-architecture bottleneck cartridges, as does the Hornady LNL. Each or either gives a “real” headspace number (although it’s not perfectly congruent, without mathematical manipulation, to the figure from a headspace gage used for chambering; that doesn’t matter though: as long as the gage is zeroed it shows the difference, and that’s what matters). By the way, the old standard “drop-in” style case gages might keep ammo safe, but won’t provide this sort of detail in information. The numbers we need to get from our gage are these: new, unfired case shoulder height (where we started); fired, unslzed case shoulder height (where we went to); sized case shoulder height (where we need to get back to).

To be rechambered, this case has to have its case shoulder “set back,” which means that the sizing die has to contact the shoulder area enough to budge it, bump it, down to a tolerable height. Here next is how to find out what that “tolerable” height is.

The process of adjusting a sizing die to produce correct cartridge case headspace is plenty simple and easy, and requires a specialty tool (and you knew that was coming): a gage to determine datum line height.


First, and important: this has to be done on the first firing of a new case, either a factory-loaded round or your own creation. For more conclusive accuracy, measure 4-6 cases, and, very important: de-prime a case before taking a read (the primer might interfere).

Measure a new case. Write that down.
Measure your fired case. Write that down.

Again, in a semi-auto the chamber might not actually be as long as the fired case reading says it is. In a bolt-gun, the post-firing case headspace dimension is going to be a closely-accurate indicator of the chamber headspace (but always subtract 0.001 inches from any reading to account for the predictable “spring back” in brass).

headspace reading
New — 1.458 inches.
headspace reading
Fired — 1.464 inches
headspace setting
Die setting — 1.460 inches.

To set the die, take the fired case reading and reduce it. How much set back? I recommend 0.003-0.004 inches for something like an AR15 or M1A. That’s playing it safe, considering, again (and again) that there may likely have been additional expansion beyond chamber dimensions. I’d like to see folks set back their bolt-guns at least 0.001, but I’m not going to argue! I don’t like running sticky bolts.

Set up case sizing die
Thread the sizing die down to touch the shellholder when the press ram is at its highest point of travel (whether it “cams” or not). Then back the die up (off) one full turn. Lightly seat the die body lock ring against the press top, and repeat the following process: lube and size the case, check the headspace; adjust the die downward, check the headspace. Rinse and repeat. For a 7/8-14 thread, which is virtually all presses, a full turn equals 0.0714 inches. That little nod of knowledge helps keep from going too far too soon, and also shows just how fine the adjustments get right at the end. When you think you got it, size a few more cases and read them. When you know you got it, lock the die ring. Note: the expander/decapping assembly was removed from this die, for one, because t doesn’t factor in establishing headspace, and because I set it all up separately on a new die. Headspace is the first thing I set.)

A little extra space ahead of the case shoulder helps ensure safe and reliable functioning in a semi-auto, and also, importantly, reduces the chance that the case might bottom out on the shoulder area in the chamber before the bolt is fully locked down. Firing residue in a semi-auto chamber is also effectively reducing chamber headspace, and that’s another reason for the little extra shoulder set-back. Keep the chamber clean!

headspace reading
Don’t just set the die bottom flush against the shellholder and commence to shucking cases! Most die makers provide that as instruction, and some say drop it down another quarter turn or so beyond that. That’s excessive. Here’s the read I got from flush die-shellholder contact on a new Forster.

Why not just set the shoulder back, for either action type, to what the factory set for the new case? Doing that really wouldn’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” that the brass has to endure, that much shorter serviceable brass life. However! That’s not nearly as bad as leaving the shoulder too high! That’s dangerous.

Bolt-Gun Only!
Do you have to do this with a bolt-gun? I say yes, but freely admit that, at the least, from zero to “just a tic” is safe enough. What you do need to do is know what you’re getting! For a bolt-action it is possible, and some think wise, to determine the necessary case shoulder set-back based on what is needed to close the bolt on the resized case: adjust the die down a tad at a time until the bolt closes. Depending on how stout the load is, it might be 2-4, or more, firings before the shoulder needs to be set back for a bolt-gun. But, rest assured, it eventually will. Just keep up with it. I think the bolt should close easily (and if you’re having issues with that in your handloads, there’s the first place to look for a cure). It’s really not possible to follow this plan with a semi-auto because the bolt will close with much greater force during actual firing. 

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.


17 thoughts on “RELOADERS CORNER: Cartridge Case Headspace”

  1. I have a bolt gun with a 23 Remington chamber, an AR15 with a 223 Wylde chamber, and another AR15 with a 5.56 NATO chamber. How do I set my headspace so a piece of brass will fit safely in all three chambers?

    1. There is two ways to address your gun chamber differences. First is chamber so the bolt action rifle loads correctly, and then test your semi autos. Second and probably best scenario is to keep the brass separate for each rifle. You want to size the whole piece of brass for your semi autos and just neck size the bolt action brass. The second process will make more accurate rounds considering all other factors like charge, bullet weight, twist in barells, and other factors.

    2. For the purpose of this post, I won’t assign any specific numerical values to measurements because there are so many chamber variations out there, any stated numerical value would be largely meaningless and/or confuse things. Then there’s all the reamer grinding and sharpening making it even more confusing, so I’ll just stick with basic, spec drawing relationships.

      The headspace for the 5.56 NATO is longer than a .223 Remington (SAAMI), so you’re better off NOT using a one-size-fits-all headspace; the 223R will end up with excess headspace in a 5.56, making your AR brass life very short, or a bolt action 223R will not close on a 5.56, and the 5.56 operates at 10% higher pressures than the 223R. Also, the .223R has only half the freebore of the 5.56 which only makes the 5.56 pressure difference even greater in a 223R.

      Then there’s the 223 Wylde, the best of both worlds. The secret is in its freebore. The Wylde freebore diameter is tighter and longer than both the 223R and the 5.56, and the headspace is somewhere in between. Therefore, the 223 Wylde will handle 5.56 pressures and is more accurate than both.

      In your case, you could: 1) Load everything using 223R data and live with the consequences. 2) Have your 223R bolt gun chamber reamed out to 5.56. There are special reamers ground just for this purpose. 3) Dedicate some brass to just your 223R bolt gun, and use the rest for your Ars. #3 is what I do.

      For reloaders who shoot semi-autos, especially AR reloaders, get a 223R cartridge case, go/no go length/headpace gage (Dillon, Hornady, LE Wilson, Lyman). It is nothing more than a 223R SAAMI spec chamber that you drop your brass into. Use it for setting your FL sizing die, via trial and error, somewhere between min and max headspace. It’s pretty fool proof.

      Use this same gage for loaded AR rounds, too. Sometimes, the neck will get pushed back and case shoulder will bulge during bullet seating, and especially during crimping. If that happens, your gage will show “excess” head space, but all that is needed is to reform the case shoulder. If you don’t do this second check it’s only a matter of time before you will get a bad jam.

      Now, I’ll make use of this forum to let off steam; NOBODY MAKES A 5.56 NATO HEADSPACE/CASE GAGE! Would somebody please make one? A 223R gage can be used for 5.56 by treating max for min, but that’s barely ‘good enough for government work’. With all the ARs out there, America’s most favorite rifle of all time, nobody makes a cotton pickin’ 5.56 NATO case/headspace gage for serious reloaders. It’s a crime!

      1. The only way to know is to check what you get in your chamber. I’ve had enough different barrels to tell you that “identical” barrels are not identical.

  2. Error on 1 turn of die giving .0174 inches on full turn of die . Should be .0714 inches on full turn of die. the .009 approx. for eighth turn is correct. (multiply .009 by 8 ,full turn, to get .072

  3. If the rifles were head spaced perfectly at the factory the same die setting would work for all of them, however I would carefully check the actual headspace on each one. They may be close enough to use the same setting for all of them.

    The difference in these chamber designs is not in headspace, but leade. The rifling will start closest to the bullet in the 223 chamber. The 223 Wylde is next, with the true 5.56X45 chamber having the bullet travel the further down the barrel before the rifling starts. The 223 Remington chamber is generally thought to have the most accuracy potential. The Wilde chamber was developed to improve accuracy in the AR15 platform and still shoot the high pressure MilSpec Ammo safely.

    I load all my Ammo for the AR15’s I have and they all have 223 Remington chambers except a couple of shot barreled carbines that have Wylde chambers.

  4. What’s your view on resizing .40 S&W cases for .357Sig handloading? I know they are slightly short on the throat that holds the bullet, but they really lock battery and fire fine in my Glock 32. Thanks for any replies.

    1. No offense, but why would you dink around doing that? The 357 Sig round is still being commercially produced. Just buy some new brass online or find a dealer in once fired brass. Life is too short for some chores.

  5. Too whom it may concern,
    I screwed up in my reply to rdsii64, in paragraph 6:

    “your gage will show “excess” head space”, should actually read:
    “your gage will show too little headspace”

    Please, make the change, or show me how I can make the it.
    Sorry, my bad.

  6. How did u arrive at 0.0174 for 14 tpi if u divide 1.000 by 14 it comes out to 0.07143 pitch.

  7. What about the bottom of the case not being sized to properly fit the chamber if we’re not fully threading the die down to the shell holder?

    1. That’s not likely to ever be a problem. There’s usually a pretty good amount near the case head that won’t show any evidence of die contact when the case is run fully into the die. Given the relatively tiny amount of gap adjustment we’re talking about in setting the headspace, that much on the case body won’t mean it’s going to hurt function.

Leave a Reply