[jboot]

You are (sort of) correct. The AR-15 platform is basically Lego for guns. The only caveat is that there is a limit to the length of cartridge. A .308 AR-15 is possible, but would not function as a semi-auto. (Indeed, there are .50 BMG uppers for the AR-15, but they are bolt action.) If you need a semi-auto .308, you’d need to opt for an AR-10 platform rifle instead, which is the gun that the AR-15 was “shrunk down” from. Once you have the lower, the sky is the limit. Pop out two pins to switch to any compatible upper in any caliber under a cerain cartridge length.

[hiredhand]

Not to split hairs, but you might find this interesting. Originally, the AR-10 was significantly different from the AR-15. It "looked" a lot like the AR-15, but it really wasn't. The AR-15 was (as we all know) chambered for .223 Rem or 5.56mm NATO and had all the improvements built into it that the AR-10 lacked.

The AR-10 that we know of today that Armalite (and others) produces is a scaled UP version of the AR-15, not the other way around.

...useless trivia... but there you have it. :-)