shoulder bump for ar rifle?

[lpreddick]

I have a Les baer AR varmint in 223 and it does as it's name says. I learned here to bump the shoulders .002-4 over fired brass. Working with new brass I frequently get very small groups that may not be there after resizing these cases. I measured new brass and base to shoulder is .004 shorter than resized brass. New brass has a good bit of headspace. I think I read that a number of AR competitors use new brass routinely... I have resized some fired cases to simulate new brass but have not shot them. What is the " consensus" on resizing fired brass for the AR?

 

[spclark]

What is the " consensus" on resizing fired brass for the AR?

It needs to be done!

Gas guns need resized brass more than bolt guns if you expect them to perform properly when feeding from a magazine. If all you want to do is single-load, it's still important - but in different ways.

What's important in the first case (sorry, pun intended here!) is that your sizing die be of a design (interior profile) that properly sizes brass for your rifle's chamber.

So you say new brass works best even with a vastly shorter headspace than what you aim for when resizing? Any particular brand work better than others? Have you tried pushing your shoulders back to match that new brass dimension? (I can't recommend this as normal practice though as by doing this you're severely overworking upper case bodies & shoulders.)

New brass is short so it'll fit almost any chamber profile. It may have a larger shoulder diameter than what your sizing die gives you though, otherwise you might find yourself suffering from mis- or non-firing rounds due to cases being too far forward when the firing pin strikes primers.

Conversely, even with headspace properly dimensioned for your chamber witn your die there may be other issues from sizing that end up giving you cases that aren't aligning properly once chambered.

First off I'd suggest you remove your bolt's extractor and ejector then set up your sizing die to run some once-fired brass. Does unloaded & unsized brass chamber under recoil spring pressure alone? If it does, you need only size your necks to hold bullets.

If it doesn't, work your die down by 1/8th turns until brass chambers easily. Minimum sizing's what your after, not some arbitrary number.

You using the supplied expander ball too? Probably necessary if your sizing die's not a bushing style, as most have neck portions of far too small a diameter thus mandating use of the expander ball. That combo overworks brass too, which is a bad thing for accuracy and case life.

 

[918v]

I bump the shoulder .004" for my ARs. They feed and eject with zero malfunctions which is surprising given I scewed them together in my living room. I run high tech stick powders, though, so they don't get dirty. If I ran H335 I'd prolly bump more.

 

[spclark]

I bump the shoulder .004" for my ARs. They feed and eject with zero malfunctions....

0.003" - 0.005" is the range suggested for AR'S, but if your empty cases will chamber with less bump than this you should be fine.

It's case body dimensions that can play hob when feeding from a magazine, particularly if you're shooting a match-dimension (Wylde, CLE, etc. You seen this info on service rifle loads? Dated but still has value.) chamber and expecting < MOA accuracy from self-feeding rounds.