I had a question that was recently fielded to me, and I've never built an AR before, so I'm kind of curious of myself.

"Are all AR15 uppers and lowers interchangeable?"

I know for the most part the answer is yes, but are there any exceptions? I remember reading somewhere that Colt's civilian rifles weren't made milspec and therefore there might be some compatibility problems with milspec parts?

Finally, can a gas piston upper work with a lower that was originally part of a direct impingement rifle?

Thanks.

Upper and lower interchangeability....for the most part ( spec. parts), yes. Older Colts used larger diameter front take down pins (but you can buy adapter pins for that) But, you cannot swap rifle to pistol caliber parts (quickly) due to mag well blocks, different fire control parts, etc.

Gas piston upper on DI lower, again...yes. As in the DI upper, the gas system is contained within the upper.

  Hope that answered it for ya.