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.