AR-15 Upper and Lower compatibility

[Mausermolt]

Are most uppers and lowers pretty much interchangeable? i bought a Rock River Coyote Carbine Upper, i dont have a lower yet because im out of the country and the prices are way to ridiculously high right now. but i have a Bushy back home thats a full size XM-15E2S 20'' model. could i just swap out the uppers?

[slugcatcher]

Most of them are. Your combo shouldn't be a problem. The Colt Spoters from the 90's used a non-standard take down pin but you can buy adapaters to use those with standard uppers or lowers.

[Mausermolt]

ok MOST of them are. tell me which ones wont be compatible with the RRA please :)

[CTS]

A mid 90's Colt with the large pins. Just as was previously stated. All others are interchangeable.

[Theohazard]

The only compatability issues you'll have will be aesthetic: the finish on your upper might not match the finish on your lower. Otherwise they will work just fine.

[Mausermolt]

great! thanks :)

[Mobuck]

You may find an occasional combination that's too tight or is looser than you like. For the loose fit, there are little rubber thingies that tension the rear pin making the fit feel tighter. For that occasional "too tight" fit, fortunately it seldom happens. I've seen some filing done on the upper but I don't condone this solution.
I was doing some mix-n-match a couple of weeks ago trying to find exactly what my older Son was looking for in a hunting carbine. When he'd picked the lower with the trigger feel he liked and the upper with the features he wanted, guess what, the fit was tighter than I felt comfortable with. A simple 5 minute job to swap fire control parts to another lower but had there been only one lower available, that would have been unhandy.