Originally Posted by CrazyPhuD
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).
I'm pretty sure I now know what my wife feels like when I talk about computers.