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.

Thanks

There are basically two sizes of receiver.  The standard AR15 receiver will work for .223/5.56, 9mm, 6.5 Grendel, 6.8 SPC, 7.63X39 and all those others that have cartridge lengths equal to, or shorter than, 5.56.  All AR15 receivers, regardless of caliber designation on the side, are functionally identical (with certain exceptions).

Cartridges based on the .308/7.62X51 require a larger receiver (and consequently a larger rifle) such as the AR10 or SR25.

The pistol calibers require the use of a magazine well block and a different buffer.  There are some companies that make removable blocks, or that use magazines that eliminate the need.

On the smaller lower you can use .223, 5.56, 5.7 x 28, 7.62x25, 7.62 x 39, 6.5 Grendel, 6.8 SPC, 450 Bushmaster, 458 SOCOM, 50 Beowulf, 6mm, .300 Blackout, and a few others I can't think of.  Think of this as a short action if you are familiar with bolt guns.

In the larger you can do the 7.62x51, .308, .204, .243, .260 and others I'm sure.

And in the smaller size you can get a single shot .50 BMG upper as well as a crossbow upper.

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.