With AK's and things, you have to remember not to run afoul of 922R parts count.IANAL but purely on the concept of reassembly, pretty sure that is legal. I recall import cut up AKs being brought into the country and being reassembled and made into rifles. But those were not NFA items.
But pretty sure you cannot make a transferrable MG out of them. That is a bridge too far.
Threshold question: are you an SOT?
You can use the parts and make a semi-auto version. You can not legally reassemble it into a machinegun (unless it's papered as a DeWAT, but that's a whole other story).