Take this for what it's worth. I've got two ARs that are fairly accurate relatively speaking (~ a very consistent .65-.75 MOA). Previously I'd been using your standard Redding FL sizing dies (leaving the expander in) and bumping ~.006-.007 in this specific AR15. I recently tried an experiment of loading for it the same I would my bolt gun (neck turned LC brass, bumped the shoulder .002, bushing FL sized .003 under loaded diameter, and mandrel expanded up for .001 neck tension; all brass perfectly trimmed in Giraud) and didn't see any difference in group size or group consistency. I also didn't have any problems with bullet setback or any drama associated with auto/mag loading; go figure.
I'm confident this barrel would be garbage in the service rifle comp world, but it's a shooter relative to most of the ARs that I encounter in the wild. That said, if you're running a Krieger or Bartlein etc. it may be worth trying some of the methods that are listed in this thread, but it's my opinion with ARs they just aren't as sensitive to brass prep, or if they are, that's all vastly over-shadowed by barrel quality, gas system setup, buffer/spring setup, BCG etc.
Basically, with a high quality bolt gun I've been able to see differences on paper with hyper-anal reloading practices; I haven't gotten those same results with my ARs.
Lastly, you can do all the brass prep you want in the world, but if the gun isn't setup right, it's all wasted effort. I've had a few AR10s in .260; one of which was way over-gassed (rifle length gas; not extended). That thing would literally deform case heads on new Lapua brass trying to rip it out of the chamber, using relatively tame load data.
Edit: This was mentioned in another thread today about custom dies, but it bears repeating. Custom dies are great, but they'll be setup for brass out of that gun/from that reamer. If you've got a reamer that you'll use to chamber barrels until you die, great. If not, it might not be a wise investment.
