I have a KAC SR25 and FN Mk20 (and Mk17 and Mk16 SBR's for that matter...) The SR25 is really nice, especially with the factory QDC can. The quality is just obvious and it has that "feel". The Mk's ain't AR's but I'd consider them for 7.62 options and you can change barrels really easy on one. For a 7.62 with 13-16 inch barrel the Mk17 is hard to beat. Reliable, compact and lightweight. Leave 'em stock and don't fuck with 'em and they're all performers. The HK 7.62 would be another good option.
All are ambi, KAC is rear charging, FN is side charging. FN 16/17 has piston and collapsible/folding/adj. cheekweld stock, solid single stage trigger worth leaving as-is for what they do, 20 has one of the best factory adj. stocks. Both SR25 and Mk20 have great 2 stage triggers. If you made me pick between SR25 and Mk20 my shit would literally crash --I probably couldn't give either up. And 16/17 both earned their spots in the safe.
SR25 and Mk20 both available in 6.5, can get uppers for SR25 and barrels for Mk20 (at least in theory you can --FN is notorious for promising barrels and parts and such and not coming through-- haven't looked for one so can't say for sure about availability). I never adopted 6.5 so all my junk is 7.62.
KAC and FN mags both are quality, so there's that too. KAC mags may be best factory 7.62 AR mags, period, FN are basically redesigned FAL mags to fit an AR-esque lower so those speak for themselves.
Mk16 is 5.56 but worth honorable mention, if for no other reason than the fact you can actually modify the Mk17 with a 5.56 upper for essentially the same rifle --but the availability of kits? Can't say. PMM may have 'em. Somebody did last year but I can't recall who.
But if I were to get another rifle today, it'd probably be the Sig Spear --probably for no other reason than it's the new infantry rifle and that blue cord in my closet just demands it. Figuring on maybe even selling my DTA .50 kit with Elite can and USO 5-25 to fund/make room for it! Only fired it a couple times anyway. That 6.8x51 looks like a real badass load too, especially coming from a 13"bbl. If it ends up being like other Sig rifles then they'll probably have a 7.62 upper sooner or later if they don't already. It ain't cheap though. Double the cost of an SR25 or Mk20 (factory SBR and suppressor but still...) Fully ambi, side and rear charging handles, collapsible and folding stock.
