I own a M1A Supermatch and find it to be pretty darned accurate. Does it have ammo preferences? Sure, most rifles will shoot better with a specific weight/manufacturer bullet. For bench shooting I prefer the M1A over the AR10 type rifle. In the field the both are going to be on the heavy side. I also own an HK91. Is it as accurate as my M1A? Not even close, but it is a reliable battle rifle and shoots well within minute of man, or minute of deer for that matter. Since I'm shooting mostly surplus with it the fact that brass gets battered and pitched from here to eternity doesn't concern me. As for an AR type .308, of the two you mentioned I'd get the RRA. Great rifles for the money with the NM trigger. Personally I'm waiting for HK to start selling its MR762 later this year. To the best of my knowledge the AR15 parts are not interchangeable with the LAR8, or AR10. For all around plinking and especially varmint hunting I use a scoped AR15. Mine happens to be a Stag Arms Model 8. I also have a RRA LAR15 which I use for personal defense. The AR15's lighter weight is a benefit when in the field shooting ground squirrel, or predators like the coyote, or bobcat. The ammo makes for a much less expensive plinking and target shooting rifle IMHO. What does all of this mean? It means you really need to own one of each. Why short change yourself?