Quote Originally Posted by mic2377 View Post
Long action cases are not compatible with the 308 AR platform, end of story. No LR-308 or AR-10 is chambered in these as they do not fit in the mag. Hence the use of 308, or one of its variants, from 243 win to 338 federal.

I would stick with 308 for any barrel length 20" or below. The 6.5 variants are also very versatile and flatter shooting than the 308 but really need a 22" or longer barrel.

Perhaps the best choice of all for most use is the 6.5 or 6.8 AR-15 cartridges. Much lighter and more compact than a large frame AR.
The exception to this is the DPMS G2 series of AR-10 rifles with their redesigned reduced sized AR-10 system. I replaced the fixed rifle receiver extension and stock on my G2 Hunter with a carbine extension and Magpul CTR stock. With the 20 inch barrel it weighs in at about 7 1/8 lbs. It is perhaps an inch longer than my 16 inch barreled AR carbine since it has no muzzle attachment. Receiver width is is just slightly wider than a 5.56 AR. I have always avoided DPMS but this is a game changer. It has been perfectly reliable and is very accurate. It replaced my lightweight .300 Blackout and I couldn't be happier.