mr kag, the ar-15 is a semi auto version of the original m-16 rifle (full auto)designed for the us military. i am retired military and have shot and qualified on the m-16, and some later varients. i did have an extremely detailed synopsis of the entire development of the weapon at one time. it is compact and light, designed for the 5.56 mm (.223 cal rem) cartidge. i have not heard of any larger caliber models of this design. I may be wrong. i have shot the m-60 rifle at a range while in the service. this gun (from what i remember) is 7.65 mm, which i remember as .308 caliber. the m-60 is substantially heavier and i cannot imagine a weapon as light as an ar-15 handling this round. currently i own a ruger mini-14 in stainless that is 5.56 mm. it is a much heavier weapon than the ar-15, based on the old m-14 design. it is my anti-piracy weapon for offshore fishing. i prefer the mini-14 due to stainless constuction in the offshore salt environment. if you are looking for a larger caliber rifle, J&G Sales sells numerous ak-47 type arms. i have bought ammo from them, but not any firearms. be safe. jeff.
The rimmed Russian round is not conducive to extended box magazine applications. The AR-10 action is not long enough for the X54R cartridge. The Cobb rifle is available up to .30-06 (if they actually are able to deliver any rifles at all). It would theoretically be able to handle this round. Mags would have to be re-engineered to work. Not worth the trouble IMHO
Currently the most versatile semi-auto platform on the planet, there are none however made in 7.62 x 54R. But there are many other caliber availabilities in the AR-15 design, the most common large caliber being the 7.62 x 51 (.308) which is almost identical ballistically to the 7.62 x 54R. If you want a real thumper look at the RRA in .458 Socom.
So does Springfield and anyone else that makes a .308. Keep in mind that the .308 can shoot a 7.62 NATO round (A 7.62 shouldn't be shooting a .308). It's the opposite of the 5.56 and .223 where the .223 can't shoot a 5.56 NATO.
So, most manufacturers just advertise their .308's.