Will an AR-15 upper work on an M4 military Lower?

well i think understand what ya want to do. it sounds like you got a lower and want buy a upper. I wa thinking of doing the same thing. going and buying a lower for 250 then buying a used upper for 350-500 depending on caliber like 6.8. i like the idea of switching uppers. you know you can get a 50bmg conversion barrel for 1500 cash. Imagine going from 223, to 7.62 39 and over to 50bmg in the matter of mins. i think its all legal