I'm no expert, but the word on the street, is that 7.62x39 is not a good mix with the AR. The advice is to go with 6.8mm instead. As an after thought, you can pick up an AK for the price of an upper and your mags will be cheaper. Just my $.02 and I'd go to the AR15 forum.com for more expert advivice. If it were me, I'd rather drop 5 or 6 bills on an AK and some mags than spend the same amount on a 7.62x39 upper.
FQ13