I am preparing to load new LC 5.56 brass and have checked length on my first 100 cases, all were proper length so then I was checking head spacing against some once fired and not resized.  Wanted to see if I needed to resize the  new brass.  They vaired from .002 up to .005 under the fired.  My understanding is that when firing from a AR I should have .003 under fired brass for proper headspace.  Will the extra .002 under be a problem?

Nope, GTG. Not a bad idea to run them through a sizer first, but if the necks are ok, your ok.

I'm confused.

How are you checking headspace and with what tool are you doing so?

Measuring case length is not the same as measuring case heaspace, just to be clear.

You need headspace gages along with a dial caliper, to measure headspace.

The rule of thumb for semi-auto gas guns is .002" to ~.004" of shoulder bump, so you'd be fine if you're in that range.  This is for new/fired cases in general.

If you go too much more than that, your cases will be stretched upon extraction, a bit more and may lead to case head separation down the road.  

Chris

Used Hornady bump gauges.  That was my worry that a few of my new unsized brass already had to much head space.

First off,

Welcome to forum.

Secondly,

I bushing-bump neck size new brass.

Neck tension is inconsistent with Lake City brass.

It's not a bad idea to bushing-bump size LaPua either.

I kind of figured that was the problem.  Just shoot them all and then set your die up for that .002"-.004" bump.  If they don't last 5-10 cycles, oh well...it's not like you're spending $2 on a case like the 338LM.

This is why I don't go berserk on my AR-15 fired brass––it's probably not going to last all that many cycles.

Chris