Headspaceing for the AR

[Blood and Bones]

My first post.

I have been reloading pistol and bolt guns for years. I now have a AR10 and now need to understand how to bump the shoulder to the correct measurement. I'm being told that I cant use the fire formed brass because the case is still expanding during its extraction so the headspace will be a lot longer than what the chamber dimensions really are. What's the best way to determine my headspace? Do I size the brass and than load up a round without primer/powder and slam the bolt down on it and measure the headspace, and keep turning down the die until it doesn't change. How do you determine your headspaceing without slugging it.

 

[adcoch1]

Welcome to the High Road! What you need is one of those reloading headspace cases that all the big reloading houses sell. Its a regular brass case that is properly sized to standard dimension's to give you a starting point. Kind of like a go guage for your reloading die. But I cannot for the life of me remember exactly what they call them. Someone will be along shortly with answers...

 

[Blood and Bones]

Can I measure a unfired Lake city XM80 mil round and use whatever they have there headspace set at. Im using a Hornady comparator to set headspace. Don't really want to work the brass to much. Im always reading about these guys who bump there's back .003 - .004 , but how did they determine what that is without using fire formed brass?

 

[243winxb]

See if fired brass , from your rifle, will fit back into your chamber and let the bolt lock.

If it chambers, measure with the Hornady comparator to get head to datum measurement.

If it does not chamber, bump the shoulder back till it does.

 

[Bart B.]

Use your Hornady comparator on a new cartridge case, then set the die in the press to full length size fired cases that read the same. Turning the die 1/10 inch on its circumference moves it up or down about .002" in height on the press.

Once you have fired cases from the rifle, reset the die to bump fired case shoulders back .003". There'll be a few thousandths spread across resized case headspace. That's normal and same as new ammo.

To make precise die settings, look at post 9 in:

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

reloading headspace cases

Never heard of that. A link, please.

 

[Wreck-n-Crew]

Full length size them. No neck sizing and no bumping shoulders. That's the best way for a Semiauto IMHO for reliability of feeding.

 

[1stmarine]

Full length size them. No neck sizing and no bumping shoulders. That's the best way for a Semiauto IMHO for reliability of feeding.

Hi.

I think this is the best method if one is going to use the ammo in several firearms and those firearms are withing the acceptable tolerances.

But lets say one has a firearm on the loose side regarding headspace or one simply wants to make sure the brass lasts potentially longer given all other considerations.

then it is ok to control the case length and therefore the amount of headspace relative to that firearm to be used in that firearm only in controlled conditions and nothing like critical duty.

So IMO, in this case the general rule can be avoided and this way achieve what one wants no different than a bolt action, lever action or any other firearm.

 

[Bart B.]

no bumping shoulders

If they ain't bumped back, they may not chamber easily as AR bolt faces ain't all that square

What if the die's set to not touch the shoulder? The case's headspace will be longer after sizing.

Case headspace is measured from case head to some reference diameter on its shoulder.

Not from bolt face to case head when the round's fired. Both Redfield does now and Hornady used to say this Incorrect stuff in their web sites. "Head clearance" is the space between case head and bolt face.

 

[1stmarine]

See if fired brass , from your rifle, will fit back into your chamber and let the bolt lock.

If it chambers, measure with the Hornady comparator to get head to datum measurement.

If it does not chamber, bump the shoulder back till it does.

This is ok and simple.
I would add if one wants to try this first to really find out in fine deatil one might take out the bolt ejector plunger and play with this simple
method you described fishing the brass from the chamber and do it with a very clean chamber just like if it was a headsapce gauge.
So one can get a feeling w/o ramming the brass fighting the ejector pressure.
Always write down the values from the comparator from fired cases, and rounds known to be reliable and/or perhaps accurate.
Needless to say never swap bolts in similar firearms even if they are from the same model/manufacturer there is not guarantee the are the same.

 

[Wreck-n-Crew]

If they ain't bumped back, they may not chamber easily as AR bolt faces ain't all that square

What if the die's set to not touch the shoulder? The case's headspace will be longer after sizing.

Case headspace is measured from case head to some reference diameter on its shoulder.

Not from bolt face to case head when the round's fired. Both Redfield does now and Hornady used to say this Incorrect stuff in their web sites. "Head clearance" is the space between case head and bolt face.

I meant use a full length sizing die and full length size instead of just bumping the shoulder back or what some call Partial full length sizing ( how can that even be both? ).

 

[Bart B.]

Partial full length sizing bottleneck cases typically ends up pushing the shoulder forward as the body's squeezed down. It only "partially sizes" case necks and bodies, not all as full length does. These cases so sized usually bind the bolt a little so it doesn't go into battery exactly the same each time. That guarantees less accuracy.

 

[adcoch1]

https://www.midwayusa.com/product/4...all-length-gauge-modified-case-308-winchester
And I was incorrect on how it is to be used. Just one of those days. I was under the impression that these were to be used for both cartridge shoulder setback as well as distance to the lands in a barrel. Sorry for the confusion...

 

[Walkalong]

Partial full length sizing bottleneck cases typically ends up pushing the shoulder forward as the body's squeezed down. It only "partially sizes" case necks and bodies, not all as full length does

Not if done correctly, at least not for what I call partial full length sizing. It couldn't be full length sizing, even "partial", if the shoulder isn't moved at all, or worse, moved forward.

It is a mistake people make though, not sizing enough with the full length die to move the shoulder a little when trying to "partial" full length size. Which is why we need to measure where the shoulder is before and after.

But there is no definition anywhere of partial full length sizing. We could argue that anytime we size the body and move the shoulder we are full length sizing, and that even if we do not size as much as the die allows we are still full length sizing if we move the shoulder any. But in general I consider full length sizing less that the die will allow "partial" full length sizing.

 

[Bart B.]

Herein lies the language issue at hand. Depending on ones perspective, they'll call it as they see and think it. SAAMI has a glossary all companies subscribing to its definitions but sometimes ignore the fact they've agreed to its terminology. They use different definitions that better suits their opinions. Most common are headspace, throat, leade and freebore.

The several die making firms I've asked about full and partial sizing all agree full means reducing all fired case dimensions forward of the pressure ring. Partial means some parts of the case are not reduced and some may increase; case headspace, for example.

 

[Blood and Bones]

Wow ! So many answers. Ive been on another unnamed site for years and the only answers they ever gave me was smug BS. It was like you needed to be in there click or you didn't deserve a correct answer. I think I might stick around if you don't mind.

Back to my problem and some info on what has been happening:

Gun is a RRA AR10
used fire formed brass that measured 1.624 (three different headstamps, L.C., PMC, WCC) and FL sized and bumped it back to 1.620.
Loaded it up in the mag without powder/primer, just the bullet seated at 2.800.
Slammed to bolt forward and then ejected and it measured 1.613 on all three.

So your saying that using fire formed brass as the baseline for bumping the brass back is incorrect due to the brass still expanding while its also ejecting is giving me a false
baseline on which to measure for shoulder set back. So do I just partial FL size it and not bump the shoulder back?, or do I FL size it and bump the shoulder to say 1.613 based on the numbers Im getting from above with my comparator?

 

[Bart B.]

Could that be 1.624 inches because your whole number thing was off by 2?

Fired case headspace from semiautos can be a thousandth longer than its chamber headspace.

I suggest your full length size several cases such that they've got headspace increments in .001" from 1.630" to1.640". Then strip the rifle so each one an be chambered by hand gently closing the bolt on each on in dimension order short to long. The two whose first chambers and second doesn't shows the chamber headspace to be between those numbers. Full length sized case headspace should be about .003" less than that.

 

[Blood and Bones]

I just pulled out and tried a factory loaded:

L.C. XM80 mil round - 1.618 unfired out of the box and after slamming the bolt it measures 1.614
Fed premium 168 SMK - 1.619 unfired out of the box and slamming the bolt it measures 1.613

 

[Blood and Bones]

Im using a Hornady headspace compatarator. So it added 2 inches to the number. So yes it really is 1.624. went back and fixed it.

 

[243winxb]

See if fired brass , from your rifle, will fit back into your chamber and let the bolt lock.

Without SLAMMING it.

 

[Blood and Bones]

Without SLAMMING it.

No it doesn't fit. Doesn't lock.
I know I will need to FL size it. Just a matter of what to set the shoulder to. Im thinking that slamming the bolt down on it gives me a consistent reading of 1.613, so maybe that's where I need to set the shoulder at so the headspace doesn't change when the bolt locks down.

 

[Walkalong]

The several die making firms I've asked about full and partial sizing all agree full means reducing all fired case dimensions forward of the pressure ring. Partial means some parts of the case are not reduced and some may increase; case headspace, for example.

By that definition, and I'll accept their take on it, there is no reason at at all to "partial" full length size. IMHO of course.

 

[Kaldor]

Myself, in an AR platform, "bumping back the shoulder" is a waste of time. These guns are designed to have full length sized rounds fed through them. Also, worrying about getting your bullet in exactly the right spot for OAL, as in "ten thou off the lands" is generally impossible as you simply dont have enough mag length to accomplish this. Also, guns like AR10s are generally heavily over gassed. Solution for that is to use an adjustable gas block. If you are ejecting anything in front of 3 oclock, and you have marks on your cases from the extractor, thats a good indicator that you need an adjustable gas block. Some people mess with springs and buffer weights, but the gas block is the best solution to fix over gassing.

In a bolt gun, yes, bumping the shoulders, and setting bullets to the perfect off the lands measurement is the reality for the most accurate shooting. Not really applicable in an AR.

 

[Walkalong]

I have no experience with .308 in an AR, but move shoulders .003 for 300 BLK with good success. I size to fit a case gauge with .223.

That said, if you just full length size per the die instructions, or size to fit a case gauge like Wilson or Sheridan (Better), you won't have to worry about function, case life will be nearly, if not as good, and accuracy will be just as good either way. All assuming a proper chamber with good headspace. Some people even like to use small base dies to size for auto loaders.

And I agree with Kaldor, load to fit the mag, no need to chase lands.

 

[Blood and Bones]

Thank you guys for your input.
I have decided to FL size and bump the shoulder .004 and be done with it. I guess im making too much out of how much to bump the shoulder to get the most firings out of a case.

 

[Bart B.]

in an AR platform, "bumping back the shoulder" is a waste of time. These guns are designed to have full length sized rounds fed through them.

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