The AR is modular and 7.62x39 is the cheapest ammo that Cabela's carries.
Nice thing about the AR is you can plug 5.56mm or 6.8mm in.
I agree that getting the buffer tube area right could be tricky.
I'm currently in the process of setting up a CNC mill similar to this one.
(Just barely big enough to cut the buffer tube area...)
As long as you get your workpiece zeroed out properly and you tell the computer to mill the hole and threads in the right place, it should come out right.
Most 7.62x39 is imported and will be among the first cut off by the Obamination.
There is little US 7.62x39 production (though it is growing) - there is a LOT of 5.56 NATO in the US.
Also, unless you do a dedicated AR for 7.62x39, you are likely to end up with a jam-o-matic - so you’re going to toss out the modular thing or the reliable thing, one way or the other.