Head-spacing for autoloaders

[gdbleb]

I've been shooting Highpower for a few years and thought I had this issue figured out until my son bought a R700 5-R and we had to recalibrate the .223 FL bushing sizer to those fired cases.
We were chambering and resizing and chambering and gauging and measuring and comparing and I discovered to my horror that when chambering a resized case in the AR the shoulder is setback .003 - .004. No firing here just chambering. Granted this is a relatively violent operation.
I've been told this is "normal" but I wasn't comfortable with it so I have subsequently increased the setback for the brass for the AR to .004.

Accuracy (and scores) have improved.
Is this indeed normal practice?
I'm concerned now about too much setback.
I know most of you folks are BR bolt folks but I've found this forum to be more quantitative and thus informative.

 

[bigedp51]

gdbleb

The majority of AR15 rifles are over gassed, in simple terms the bolt can be moving to the rear while there is still pressure in the barrel. Therefore a fired ejected case can have a "longer" cartridge headspace than the chamber because this pressure can move the shoulder forward. This means a fired case can give you a false "chamber" headspace reading with the cartridge headspace being longer than the chamber.

I have three full length .223 resizing dies and these different dies when the press is adjusted to cam over will make the cases .007 to .009 shorter than fired length.

If you want to double check your chambers actual headspace you can use a new or full length resized case and a spent primer to get your actual chamber headspace length. You place the spent primer in the primer pocket just using finger pressure and then chamber this test cartridge case and let the bolt seat the primer.

The head clearance is the "air space" between the rear of the case and the bolt face. By measuring the cartridge headspace length with the Hornady cartridge headspace gauge you will have your actual chamber headspace reading. And then shoulder bump can be anywhere from .002 to .006 on your resized cases. You can also use the primer trick with your normally resized cases and check primer protrusion. If the primer is flush with the base of the case, that is a good sign your fired cases are longer than the chamber and you need more shoulder bump.

A full length resizing die at minimum will make the case .002 shorter than the GO gauge to ensure all full length resized cartridge will fit any chamber. I say this because many reloaders just set the die up as per the instructions without any thought about minimum shoulder bump.

That being said many rifles will give their best accuracy when the cartridge fits the chamber like a rat turd in a violin case.

 

[searcher]

When setting headspace for a bolt rifle - I want my setback (if any) to be no more that .001"

In my semi-autos (A/R's in .223/.556, BAR in 7mm Rem Mag and M1-A in .308), I set the shoulder back just over .001" to provide just a bit more leeway. I have noticed minor shoulder set-back on a couple of guns but, because it did it throughout my load tuning, once I found my best load, it still worked just fine. In essence - as long as it was consistent - which it is - no problem.

Personally, I would be inclined to NOT set the shoulder back further as you did. In my thinking, this just creates more unnecessary slop, likely increasing inconsistency of actual headspace conditions when firing and creating much more case stretch, shortening the life of the cases and prompting more trimming. That said - if your rifle shoots better doing it....... Kinda like shooting factory ammo I guess.

 

[bigedp51]

When setting headspace for a bolt rifle - I want my setback (if any) to be no more that .001"

Your a prime candidate for neck sizing dies, the OP has a AR15 semiautomatic rifle and its "NOT" a "bolt action"rifle. And there is a reason why military chambers are fatter and longer than commercial rifles.

Right now I'm resizing all my once fired .223/5.56 brass I have with a small base die and the press reaching cam over. I guess you have never bought once fired Lake City cases that were fired in a M249 machine gun.

 

[searcher]

Biged51 - You should have read past the first sentence in my post - the part about semi-autos. Most of my semi-autos are also A/R 15's, mostly match rifles. Your comment about my being a candidate for using neck dies (presumably because I use bolt rifles) is, indeed, well-founded. I use them in all of my benchrest rifles for competition - as well as in ALL of my semi-auto rifles - including the A/R-15's which I also use in competition. Nothing, in my opinion, beats the Redding full-length "Type S" bushing dies for the .223 in an A/R for accurate ammo. I should note that I don't load my ammo for combat (my Marine days are long over) and as such, I don't crimp my ammo, I load to minimum headspace for semi-autos, don't use crappy military bullets and don't use bullets with canellures. Having loaded many, many A/R's for 40 years, it didn't take too long to find that ALMOST ANY A/R would shoot DRAMATICALLY better once the brass was fire-formed to the chamber and tight headspace was applied to the reloaded product. While we all know this is true for all guns in general - it seems much more pronounced in semi-automatics (especially those with military heritage). There is a LOT of headspace inherent in many of these as opposed to many other calibers. Adding to the mix - you have manufacturers selling "minimum-dimension chambers" - not to mention the .223 Remington, Wilde, and 5.56 - all of which have deviations within their own. To think you can stick a case in any of these and have properly headspaced ammo is questionable - as best.

 

[markm87]

I never attempt to get cute with shoulder bump when it comes to loading for an AR. I simply set my FL, small base, carbide die to where it sizes ammo to a chamber guage.

I load 30 cal ammo cans 500 rounds full of 77 gr SMKs at a pop. The rounds must be able to run in anyone's AR.

I've done this for 20 years or so, and have had 4 or 5 case separations. I've found that the primer pockets let go way before case separation becomes an issue.