I have a Bushmaster AR15, but I shoot left handed. Stagarms makes a left handed rifle. Can I just buy the left handed upper reciever and switch them out? From my understanding... that's the only part of the rifle that is different. Is this true?

While I think that to be true I would also consider just buying a brass deflector. The only thing about left hand rifles is their expense. If you already have a right handed rifle and could get a brass deflector, they why spend the money for a new upper reciever?

I would say just stick with the right handed one. But I would imagine that it would fit. You could call the company and ask them to make sure.

I also suggest calling the company to be sure. If you're talking about a stripped lower, then I don't think it will work at all. The bolt will try to eject the brass out the wrong side, but maybe not. If you're talking about a complete lower with the bolt carrier group installed, then yes it should work just fine.