Originally Posted by kepp
So what is the difference between the AR15, M4, Colt A2, etc? They all look the same and seem to be .223 caliber.
If they are all milspec then fundamentally 95+% of the parts will be interchangeable. I believe some of the colts have larger pivot and takedown pins which prevent them from being 100% compatible. But most of the AR-15(semi-auto M16 which all of them fall into) can fully swap parts. That's why I have fun building mine rather than buying off the shelf. You get to cherry pick which parts go into the gun. AR-15s in rifles and 1911s in pistols both allow a TON of customizations if you want.
Interestingly enough while the AR-15 platform is almost 100% standard the AR-10 platform is fairly fragmented. Part of the reason I got the AR-15 was all of the support. I'm planning to have multiple uppers, 5.56, 22LR, .410, 5.7x28, and likely a .50 WSSM variant(but you have to reload that).