Headspaceing for the AR

[243winxb]

bump the shoulder .004

From what number? 1.613"

1.624" fired brass to 1.613" + .004" more = .015" stretch? Not good.

I still think the shoulder should be set back so the bolt just closes for a correct measurement. No slamming.

What if the extractor and ejector push the cartridge deeper into the chamber, then the bolt face, making the head to datum shorter?

What if the bolt has over travel, when slammed?

But i dont have one. So , good luck.

 

[Bart B.]

Firing pin impact often drives cases deeper into chambers. I've measured .007" shoulder setback on Federal nickel plated 308 Win cases, 003" on all brass cases. How much depends on shoulder angle, area, metal types and surface finish along with firing pin spring and primer cup strength.

Firing pin impact setting case shoulders back was why rimless bottleneck cases over a hundred years ago with a small shoulder angle had a belt put on them to stop that.

 

[Kaldor]

I agree that these guns are designed to have full length sized rounds fed through them. That's what new cases are; full length sized from the get-go.

If a new round has a case headspace of 1.464" and fired in a chamber with headspace of 1.468" and ends up with a 1.467" headspace, what's wrong with full length sizing it bumping its shoulder back .003" to 1.464"?

Note SAAMI spec for .223 Rem chamber max headspace is 1.4736" and minimum is 1.4636" Spec for new case minimum headspace is 1.4596".

AR10's in 308 are no different; just bigger numbers in the 1.626" to 1.634 range

I just dont feel that screwing around with it worrying about a couple thou in an autoloading action is worth the trouble considering that these pieces of brass take a huge beating, unless the gun is tuned properly. And even when tuned just right, the brass life is way less than a bolt gun. The other side is this as well, I own multiple guns that shoot 223, and I want bullets to run in my AR as well as my bolt 223, so full length sizing, including bringing the round back to SAAMI spec is a must for me. My buddies also shoot my reloads from time to time, so once again, full length sizing is a must.

Now, on my bolt 308, I bump the shoulder 3 thou, and load em up, but if I ever bought another 308 and wanted interchangeability between the rifles, then Id go to a full length resize.

There are honestly bigger things to worry about that are far more important when it comes to accuracy that if your brass is full length sized or bumped back a couple thou.

 

[Bart B.]

If one uses a full length sizing die to bump the fired case shoulder back only .0001", or as much as die against the shell holder bumping the shoulder several thousandths, it's all full length sizing. The full length of the case is sized some amount.

Full length sizing has never been limited to only when the shell holder is pressed hard against the die's bottom. But this is what some say it is.

Which SAAMI spec should a die size a case to; minimum, maximum or can it be anywhere in between?

 

[Kaldor]

If one uses a full length sizing die to bump the fired case shoulder back only .0001", or as much as die against the shell holder bumping the shoulder several thousandths, it's all full length sizing. The full length of the case is sized some amount.

Full length sizing has never been limited to only when the shell holder is pressed hard against the die's bottom. But this is what some say it is.

Which SAAMI spec should a die size a case to; minimum, maximum or can it be anywhere in between?

Well, Im not sure which SAAMI spec it should be to be honest, but I would expect middle of the road to figure where the exact location of the shoulder should be. The most important thing to me is consistency. If I full length size my brass, to full cam over, everytime, then I know where Im at everytime. That number can definitely be different depending on the user, dies and shell holder. Its kind of like neck sizing. Yes, it works the brass less, but honestly, Ill pass on it for the same reason I dont bump back the shoulder on a case, I want my rounds to work in every rifle. Also, not sizing to full cam over could case some other issues as the base of the case may not sized as much as it should be on a tapered case.

All my point is raising is that its not worth screwing around worrying about bumping the shoulder back in an autoloading rifle.

 

[Bart B.]

Every time a fired rimless bottleneck case is sized in a full length die to full and complete cam over of the ram linkage, its shoulder is moved, pushed or bumped back some amount. Before the shell holder stops against the die bottom, the case shoulder is pushed forward as the die body squeezes down the case body.

The reason is the die's headspace is about .005" less than chamber minimum headspace. Full length die headspace is measured from the bottom flat on the shell holder case heads rest to the reference diameter on the die shoulder when they're touching each other. That's a common industry standard to ensure all fired cases so sized will chamber easy and safely in all barrel chambers. Cases so sized are close to SAAMI minimum so they'll chamber easy in a minimum headspace chamber.

If you put a GO headspace gauge in a standard shell holder then push it full into the die, there's a gap between die and shell holder about .005".

 

[W.E.G.]

Check this out.

Info on how to do the relevant measurements.

 

[Bart B.]

Or look at post 9 in:

https://www.thehighroad.org/index.php?threads/headspace.815278/#post-10440532

Use a 3/8" ID bushing for 30 caliber cases, 5/16" ID for 22 caliber cases. Metric spacers, too, whatever fits over case necks and touches somewhere on case shoulder.

All you're doing is measuring case headspace difference from fired to sized numbers.

 

[InfoLoader]

Rcbs sales a precision mic that may help determine your chambers headspace based on that fired case.just a tought

 

[Bart B.]

There typically a .003" spread in new bottleneck cases (loaded or empty) headspace as well as fired ones full length sized.

How drastic is that for reloaders wanting consistency in case dimensions?

SAAMI specs for case length is 2.125" to 2.260" for 223 Rem, 1.995" to 2.015" for 308 Win.

 

[Blood and Bones]

My trip to the range turned out excellent and have resolved this issue by FL sizing and bumping the shoulder .003 -.004 from my fire formed brass. No issues with feeding or extraction.

However I have a new Question that I thought I might ask on this thread rather than start a new one. On the way home I stopped at the gunshow and purchased some once fired mixed brass (.308) that the guy had already processed. He's a reputable dealer that ive purchased from in the past. Normally I buy his unprocessed, but he was out. Anyways after bringing them home and checking the headspace on them I see that some have been bumped back - .004 to as much as -.011 using my measurement for my gun. My question is how much is too much? What is acceptable or unacceptable from your perspective. Im thinking anything over .006-.007 is trash and running the risk of casehead separation or will the case neck split.

 

[Wreck-n-Crew]

Dies are a whole other issue on the topoic

My trip to the range turned out excellent and have resolved this issue by FL sizing and bumping the shoulder .003 -.004 from my fire formed brass. No issues with feeding or extraction.

However I have a new Question that I thought I might ask on this thread rather than start a new one. On the way home I stopped at the gunshow and purchased some once fired mixed brass (.308) that the guy had already processed. He's a reputable dealer that ive purchased from in the past. Normally I buy his unprocessed, but he was out. Anyways after bringing them home and checking the headspace on them I see that some have been bumped back - .004 to as much as -.011 using my measurement for my gun. My question is how much is too much? What is acceptable or unacceptable from your perspective. Im thinking anything over .006-.007 is trash and running the risk of casehead separation or will the case neck split.

FWIW if the brass your purchased doesn't have the signs of case head separation you should be fine to size them (just in case), load them and shoot them. Case head separation is different from failure at the upper portion of the brass and is due to the brass stretching and flowing forward of the case head. The brass ahead of the thick web becomes too thin to support the pressure and the signs are discernable prior to splitting in most cases.

http://bulletin.accurateshooter.com...se-head-separation-how-to-detect-the-problem/
https://www.thehighroad.org/index.php?threads/incipient-case-head-separation.734058/

 

[Bart B.]

With all the stuff you have and do wanting good (precise?) measurements, I suggest you get a 308 Win GO headspace gauge. Calibrate your Hornady comparitor to read 1.630" on that gauge. Then your numbers will be in step with real case and chamber dimensions. Besides, it can be used to set a 308 sizing die about .005" off the shell holder.

Full length sizing those short cases in a die set to bump ordinary fired ones shoulders back a few thousandths will squeeze their shoulders forward. Hopefully enough that actual case headspace is 1.627" or greater; SAAMI minimum. If that's no more than .006" less than a fired case from the rifle, I think all's well and safe.

The cases that don't grow that much could be sized in a 30-06 full length die and their shoulders will go another thousandth or two forward. They'll be OK to shoot in your 308.

 

[243winxb]

I see that some have been bumped back - .004 to as much as -.011 using my measurement for my gun. My question is how much is too much

Once fired brass from a different rifle-

More then .010" with repeated loadings would not be good. Possible case separations , after may be 3 loadings.

One reload/firing at .011" should not be a problem. Factory ammo does it all the time.

However, if factory new ammo is fired in a defective rifle, with excessive headspace, the brass may be already damaged.