Neck sizing for an AR
- Thread starter m1ajunkie
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It’s a shitty idea in a bolt gun, and that much worse in a semi. When you test fire it, make sure you bring an old cleaning rod and a hammer to free the stuck cases.
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Semi autos are chambered loose... On purpose.
When you fire form brass in a semi auto chamber and then only neck size you're asking for a really tightly stuck case.... As some have already mentioned.
You need to run your brass through a sizing die.
Bump the shoulder back and reduce the case diameter. You're probably going to have to trim it too... But that's really dependent on the wall thickness of your brass.
When you fire form brass in a semi auto chamber and then only neck size you're asking for a really tightly stuck case.... As some have already mentioned.
You need to run your brass through a sizing die.
Bump the shoulder back and reduce the case diameter. You're probably going to have to trim it too... But that's really dependent on the wall thickness of your brass.
Full length body die and bump the shoulder .002. Concentric it is your friend. That’s why I like using the lee collet mandrel die. I get sub MOA from my AR10 and it cycles just fine.
From the research I’ve done, neck sizing isn’t recommended by any of the top shooters out there. It’s universally accepted practice to body size and bump the shoulder. You’re doing right
No doubt, there will be a new wizz bang gadget in the future that is the latest and greatest 
Semi auto fired cases typically stretch ( in all sorts of directions ) just from the designed semi auto action, and cyclic speed.
Some S/A actions canbe crazy hard on brass... H&K, M1A's,
Extraction happens while still under substantial pressure.
Yanking the fired cases rim while still under that high pressure all the while the case is still expanded into the chamber walls.
Combine that with typically looser chamber dimensions , for function under less then perfect conditions.
Produces some fairly distorted cases....
And you need easy, smooth, complete , consistent feeding in a S/A.
Lots of S/A fired brass can benefit from "small based" reloading dies to help 100% chambering.
You need the reloaded cases to freely feed and fully , easily chamber, for proper trouble free function.
So neck sizing for a S/A is not something I would try.
If the Op is looking for precision improving "tricks" for a large frame AR, a better quality barrel and good trigger have yielded very good results for me and many others.
There are many more precision tuning tricks as well.
Some S/A actions canbe crazy hard on brass... H&K, M1A's,
Extraction happens while still under substantial pressure.
Yanking the fired cases rim while still under that high pressure all the while the case is still expanded into the chamber walls.
Combine that with typically looser chamber dimensions , for function under less then perfect conditions.
Produces some fairly distorted cases....
And you need easy, smooth, complete , consistent feeding in a S/A.
Lots of S/A fired brass can benefit from "small based" reloading dies to help 100% chambering.
You need the reloaded cases to freely feed and fully , easily chamber, for proper trouble free function.
So neck sizing for a S/A is not something I would try.
If the Op is looking for precision improving "tricks" for a large frame AR, a better quality barrel and good trigger have yielded very good results for me and many others.
There are many more precision tuning tricks as well.
Most of this is irrelevant for an AR, which is what the OP asked about, unless it's severely overgassed and destroying brass.
Civilian AR15 barrels are not "typically chambered loose", in fact a lot of high end barrels have tighter match chambers. That also has nothing to do with neck sizing, as long as brass from that barrel is going back in that barrel it doesn't matter how tight or loose that chamber is. The small base dies comment is completely off base, and doesn't line up with your claim of loose chambers; small base dies are needed for reliability with tight chambers, not loose ones.
It looks like there are a number of comments here from people who've never actually tried this, and only a few comments from people who have.
You can neck size for an AR, and expect reliable chambering if you're using cases previously fired in that chamber. The real problem is extraction; it's common for neck sized rounds to be difficult to extract without firing them. There is nothing wrong with trying for yourself though, as long as you keep that in mind. It is not otherwise dangerous or hard on the rifle.
I should clarify this comment - I'm not saying anyone should neck size for an AR, or any other semi auto. I am saying that it's worth understanding the real reasons not to, rather than just accepting some of the inaccurate guesses that are being posted in this thread like loose chambers, case stretching, etc. At the end of the day, the easy and good method is to just bump shoulders back .002-.004" with a die that produces minimal runout and doesn't size more than necessary (i.e. no small base die unless your barrel actually needs it). A Forster FL die is a good way to do this, and even better if you send it back to have the neck honed to work with your brass and chamber.
A carefully chosen chamber design with a correct for dimension FL Bushing sizer is as good as it will get.
That's great in a bolt action but have never tried that in an AR or auto loader.
That's great in a bolt action but have never tried that in an AR or auto loader.
You are right about the "this question concerns AR's ", I apologize for not being specific enough.
I also just wanted to point out some potential issues , rather then just saying a blanket don't do it answer.
Not all AR chambers are tight match chambers, that is my point.... The OP makes no mention of what brand barrel or chamber size... so I was just trying to help explain some of the possible issues in the event that the barrels chamber is cut badly.
As for the small based dies.. they also help resize fired cases from an oversized chamber. .. cases that have expanded so much that normal resizing dies may not resize far enough.
From what I have witnessed, most all 308 AR barrels are needlessly over gassed and rough on brass... only one of my barrels has a IMHO "practical gas port size" from the factory....the Krieger barrel.
Its gas port size is considerably smaller, then other barrels of equal length and gas port position.
And again, since there is no mention of what barrel is used, I guessed it would be a typical situation. More gas then needed.
I have tried neck sizing only from numerous S/A rifles... in the end, as many have mentioned, there wasn't any noticeable or worthwhile gain from doing it for me.
If you have examples of increased precision from neck sizing only, in a semi auto AR, I would love to see it.
I am not doubting you, but from my testing, ultimately 100% positive function was not achieved from just neck sizing.
And even shoulder bumping wasn't worth it for me... and I really didn't see any attributable decrease in group size from neck sizing.
I even tried neck turning.. not enough improvements in my various S/A's to merit the added effort for me.
Frankly, if only neck sizing S/A fired brass worked so well... IMHO everyone would be doing it.
So I guess it depends on what you are looking for.
To each his own... as Yondering mentioned.
@Yondering have you neck sized only for a specific AR ? And if possible, what were your results ?
I am not trying to be combative at all... just curious.
I guess I was just trying to pose a worse case scenario... and not knowing the OP's level of skill in reloading for S/A AR's ( No offense intended at all )
Sorry for not being perfectly precise in my comments.
For the OP try it if you want... but please keep an eye open for possible issues.
Even the "softness" of various brand brass cases can alter your chambering / neck sizing only results.
For the OP, what barrel are you using ?
And FWIW, IMHO, brass life from a S/A Large Frame AR can be helped more by adding an Adj GB , allowing for extraction at lower chamber pressures.
Well for starters, please re-read the first sentence in my comment you quoted; most of your post above seemed to disregard that.
And yes, I've tried it in 5.56 and in 308. I had no chambering issues, but was not able to extract most rounds without firing them, as I said in my first post above. Of course that's with brass previously fired in that chamber; only an idiot would attempt to use neck sized brass from a different rifle. It did give good accuracy, but wasn't worth the hassle of not being able to extract rounds. Full length sizing to the correct shoulder bump/setback for that rifle is the better method for reliability.
Note that correct shoulder bump does NOT mean just setting the sizing die for the press to cam over, unless that's what the dimensions dictate. Conventional die setup instructions can result in bumping shoulders back .010"-.015" or more with certain combinations, and that can hurt both case life and accuracy.
