Re: Long barrel short barrel
You should be able to stay pretty close to a bolt gun with an AR if you get a good match barrel and freefloat fore arm (I'd strongly recommend a Geisselle trigger as well, but that's a whole new tread). If you want to swap back and forth with no buffer/spring issues you might want to look at the 6.8SPC as well. It doesn't have the reputation for accuracy the Grendel does. But I can assure you, it'll group every bit as well as the 5.56 at any distance while bucking the wind better, and has about twice the knock down power (basiclly shooting a light 270win bullet) Mine likes the 120gr rounds, and the coyotes really hate it. It's got plenty of poop left at 400yds for a yote, well 390ish(my range finder is junk) I haven't got a yote any farther yet, but I wouldn't think twice about a 500-600yd yote. I can tap dance on a beer can at 500 and I know guys that are very pleased with theirs at 600. Another plus is that the 6.8 is designed for use in a 16in barrel, so you don't need a mile of tube hanging out there. I went with a Bison Armory 18in Recon and don't have a single complaint. The brass isn't cheap, but the Grendel and Creedmore are either. More and more companies are starting to make 6.8 ammo too, so the selection is getting pretty good for shelf ammo. Sorry to be so long-winded, I just find the 6.8 to be a great all-around hunting bullet in an AR-15 platform.
I like things that go "bang"... I love things that go "BOOM!"