Tag Archives: cartridge case headspace

RELOADERS CORNER: Cartridge Case Headspace

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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.

CHECK OUT MIDSOUTH Selections HERE

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.

NOTE: 
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.

 

RELOADERS CORNER: Problems Dies Can Have

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Reloading problem? First make sure it’s not your tools… Here are a few things that can happen, and how to correct them.

Glen Zediker

Over years and years (and years) I’ve encountered a few factory-faulty sizing and seating dies, and associated pieces-parts. It’s not at all common, but it happens, or has happened, enough that I wanted to share a few stories as to what these problems come from, and how to identify (and correct) them.

As has been my norm here of late, yep: got a phone call from a fellow having problems with a new 28 Nosler. Took a while to get through this one… Turned out that the sizing die was the culprit. Wasn’t easy to sleuth but there’s a Zen tenet that paraphrases to this: If you’re not sure what something is, then carefully consider all the things that it is not; what’s left is the answer…

forster concentricity fixture
A concentricity fixture is a handy tool to help identify any issues a sizing or seating die has.

By the way, I’m not going to mention brand names for one good reason: I’ve seen or been presented with issues in dies from every major maker.

Sizing die problems I have either encountered first-hand or been witness to via my circle have most often been a full-length die that will not adequately set a case shoulder back where we want to take it. Conversely, it’s much more common to have a die that’s erring on the more extreme end of that, and erring toward “too much” sizing potential is logically a direction a die maker might take to accommodate more circumstances. Once the shellholder is making full and flush contact with the die bottom, that’s all she wrote. Continuing to turn the die body downward does nothing but stress the press and possibly damage the die. To get the case farther up into the die, either thin the shellholder top surface or grind the bottom of the die hisself. Neither are hand-tool operations! Get to a local gunsmith or machinist.

Look at a hair from your head and that’s ballpark 0.004-0.006 inches. It doesn’t take much at all to make the difference between smooth function and a bolt that won’t close.

loaded round runout
Check loaded rounds to point out bullet seater problems. Check sized rounds to point out sizing die issues. Removing the expander and checking neck runout on a sized round usually points out any problem with the die itself. It should spin with no to very little runout.

Check out Midsouth tools HERE

Most sizing dies are reamed one-piece, one-shot like a rifle chamber; however, that’s not always the method. Some are done in two or more steps, using two or more cutting tools. Clearly, consistency and correctness favors the one-piece reamer. Assuming that the reamer is correct and correctly used. I have encountered one die that just wasn’t concentric, body chamber to neck area. I figured that one out by sizing without the expander and checking runout, and also by finding that I could shift off-center axis by rotating the (marked) case and running through again. Normally, sizing a case without the expander in place results in a case that runs flat-line on a concentricity fixture. Reason is primarily because any inconsistency in the case neck walls get “pushed” to the inside case neck. But if there’s wobble in a case that’s been sized sans expander, then, son, you got a die problem.

A bent or bowed expander stem will, not can, result in an expander that’s going to cock the case neck one direction. I watch for that when I polish the expander button. As described here before, that process involves chucking the stem (lightly) in an electric drill and spinning the ball against some wet emery to give the ball a shine. If it’s wobbling during this operation, that’s a problem.

polished expander
With one particular brand of bullets (which were a tad amount smaller diameter than usual) I experienced inadequate bullet grip, insufficient to retain the bullet in the case neck on magazine-fed rounds. That wasn’t really a die problem, but the (simple) solution was to reduce the expander ball diameter. The die maker replaced the stem assembly for me so I didn’t have to apply that extra tension to rounds loaded with my usual bullet brand. When you polish the expander (and you will do that right?) notice if there’s wobble. Don’t accept it.

Seating die issues, in my experience, usually revolve around plain old straightness of the seating stem, and, once, the concentricity of the reamed case body area. If you have a seating die that increases runout compared to what a concentricity fixture showed on the sized case neck, it needs looked into. Additionally, always (always) check to make sure the seating plug (the area that fits over the bullet to push it into the case neck) is deep enough the the bullet tip does not make contact with the inside of the plug. That’s a sure way to get a bullet tipped off kilter.

bullet seating stem check
Always check to see that the tip of the (longest) bullet you use doesn’t contact the inside of the seating die plug. If it does it will get skewed.

Now. Most importantly: What to do if you suspect a tooling problem? Short answer is: SEND IT BACK. Don’t accept it. I know of no maker who won’t profusely apologize and promptly return a new one. The fixes I mentioned are for those who prefer to solve such issues, and also for those who have the means to effect repairs. The point to this article mostly is to be aware that problems can and do exist, and don’t accept them, whichever direction you seek for the solution.

No matter how precisely a die maker produced the parts, there is and will be some gap in threaded pieces. This can disguise itself as a “die” problem, but it’s really not. It’s a set-up problem. I did an article a good while back here on a few ideas on improving tool/case alignment via some set-up tricks, and maybe that should be the next topic under the Reloaders Corner banner.

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.