Originally Posted By DR-B:
OK, I've been doing a lot of research to start building my first AR. I am building a 5.56/.223 but I have noticed several different calibers available both for sale complete and parts for builds. What I am curious about is if the same receivers can be used for the different calibers (i.e. .22LR, 7.62/.308, 6.8mm, .458, 9mm).
It's not going to make a difference to me for this build, but it is a curiosity if I will be able to build complete uppers in those calibers in the future.
Look up the AR-10, those are for the larger .308 and it's derivatives. The standard AR-15 can be chambered in everything else you mentioned, including 7.62x39. Also worth noting, the upper receiver itself is also not caliber specific. Typically, just the bolt, barrel, and magazine are the caliber specific parts. Some cases (like the .300 blk), only the barrel is caliber specific.
There is currently in production, but not yet for sale, a new Colt that has a lower receiver that accepts (and comes from the factory with) a larger .308 upper receiver, but can also accept any standard AR-15 upper.