newbie questions about ARs

[BSH]

I've done some looking around, and I think it's neat how ARs are pretty modular - put this upper on that lower and get a whole 'nother rifle. Just what all calibers are available for AR lowers? Can you put any of these uppers on any old basic lower? If you start with a standard Stag lower, for example, can you mount a 7.62 Nato upper on it, then change your mind and swap in a 9mm? Is it really that flexible?

[MTechnik]

Yes, mostly...

You can get a .50BMG upper for an AR-15, you just need to load it single-shot. The "big" problem you run into is bullet length.

a 9mm AR-15 is no problem, just put in a mag block, get some sten or uzi mags (whichever the kit you have works best with) and add in the 9mm upper, ta-daaa..

a 6.8spc AR or 6.5 grendel is fine, because those are made to the same length as a .223. Even the 300 grain .458 SOCOM round is good that way. You can snap an Alexander Arms 6.5 grendel upper to a colt .223 lower without a problem. You can run a colt 9mm upper on a Rock River lower, no issue, etc.

The .308 (aka 7.62x51 NATO) round has a problem where it is too long to fit in a .223 mag, so you need a longer lower. The fact that the lower is longer makes it so that the upper has to be longer as well - making for a completely custom rifle. Rock River, Armalite and DPMS all make AR-15 variants in .308. They all take slightly different approaches to solving the inherent issues, so they are not cross-compatible. I know that the Rock River .308 AR uses FAL mags for a cheap source. This limits the cheap .308 mags to 20 rounds, but I believe Rock River is making new 30-round ones. Not quite sure what Armalite (formerly Hesse) and DPMS are doing for theirs.