Cartridge Case Head Space

[survivalshop]

 Every time I read one of his articles , I have to buy more tools .  This also may be a good one to be Pinned .

RELOADERS CORNER: CARTRIDGE CASE HEADSPACE

JANUARY 11, 2018 GLEN ZEDIKER 4 COMMENTS

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.

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.

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).

New — 1.458 inches. Fired — 1.464 inches 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.

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 0.0174 inches, and an 1/8 turn is about 0.009. 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!

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.

 

 

 

[C Funnk]

 Thanks for posting the articles you post. Definitely helps a newbie like myself.

[survivalshop]

7 minutes ago, C Funnk said:

 Thanks for posting the articles you post. Definitely helps a newbie like myself.

  They help me too .

[98Z5V]

Damn.  Now I need to go play with some brass.

 

[survivalshop]

8 hours ago, 98Z5V said:

Damn.  Now I need to go play with some brass.

 

    Ya , I have to find that Gage he is using , now that the article got me curious & did you notice his photo of the Press Ram with the O-Ring holding the Shell Holder in the Ram instead of the Spring clip ? His one article & of course me purchasing the New MEC Press with the floating Shell holder , is why my older single stage press also has an O-Ring for shell holder retention , makes a difference with concentricity of the case & loaded round . 

 And I gage all my Target & ammo made for Hunters .

[survivalshop]

  That tool he is using is over a $ 100 , why is it things & tools are always around $100 , it will wait for a while , but I will pick one up .

https://www.brownells.com/reloading/measuring-tools/bullet-comparators/ammunition-measurement-system-prod55552.aspx?avs|Manufacturer_1=forster

[FreQRiDeR]

I use Hornady’s Lock n Load Headspace Comparator. It comes with all five bushings and the bullet comparator inserts are cheap so you can measure headspace and OAL from ogive as well. I paid $38 I think. 

I measured my fired brass, set the die to bump back 3 thou. Gtg. 

https://ads.midwayusa.com/product/479704/hornady-lock-n-load-headspace-gauge-5-bushing-set-with-comparator?utm_medium=shopping&utm_source=google&utm_campaign=Reloading+-+Metallic+Reloading+Equipment+(Not+Presses)&utm_content=479704&cm_mmc=pf_ci_google-_-Reloading+-+Metallic+Reloading+Equipment+(Not+Presses)-_-Hornady-_-479704&gclid=EAIaIQobChMIztni_-Ll2AIVQWx-Ch1lEQWYEAQYASABEgLIG_D_BwE

[survivalshop]

8 hours ago, FreQRiDeR said:

I use Hornady’s Lock n Load Headspace Comparator. It comes with all five bushings and the bullet comparator inserts are cheap so you can measure headspace and OAL from ogive as well. I paid $38 I think. 

I measured my fired brass, set the die to bump back 3 thou. Gtg. 

https://ads.midwayusa.com/product/479704/hornady-lock-n-load-headspace-gauge-5-bushing-set-with-comparator?utm_medium=shopping&utm_source=google&utm_campaign=Reloading+-+Metallic+Reloading+Equipment+(Not+Presses)&utm_content=479704&cm_mmc=pf_ci_google-_-Reloading+-+Metallic+Reloading+Equipment+(Not+Presses)-_-Hornady-_-479704&gclid=EAIaIQobChMIztni_-Ll2AIVQWx-Ch1lEQWYEAQYASABEgLIG_D_BwE

   

      My LE Wilson Case gages instructions read that the Case is to SAAMI specs if its between the two levels , not above or below , so its easy to test ( without a measuring device ) or set the Case shoulder set back between the two or , like I set mine , just above the lower shelf , just above the minimum spec's .

     I know I don't know the precise measurement reading , but I can surely adj. the Shoulder set back adj. & as one reviewer posted , not sure if it helps accuracy in an Semi Auto Rifle like the M1A or 308AR , maybe a Bolt Action, not really sure ,never experimented , have to take the article's word on it . 

   Have you seen a difference ?

[FreQRiDeR]

I headspace more for safety and reliability than for accuracy’s sake, as an out of spec cartridge may or may not chamber when you need it most. However, accuracy is as much about consistancy as anything else so consistant headspace translates to consistant crimp and distance to lands. This SHOULD produce more consistant/accurate ammo. Theoretically.

[survivalshop]

6 hours ago, FreQRiDeR said:

I headspace more for safety and reliability than for accuracy’s sake, as an out of spec cartridge may or may not chamber when you need it most. However, accuracy is as much about consistancy as anything else so consistant headspace translates to consistant crimp and distance to lands. This SHOULD produce more consistant/accurate ammo. Theoretically.

  I agree , the LE Wilson Case HS Gages work fine . I went for so long without using any a Case gage , its a wonder my ammo worked , it wasn't till the last year or so , I started to use them , I'm glad they talked me into using them here . I have them for all the Bottle neck Cases I reload for now .