I only have 1 experience and it was a complete Colt in 762x39, built and bought that way. Here's the kicker, functioned just fine, with U.S. commercial hunting rounds shot very well, 1 - 1 1/2" groups at 100 yds, with milsurp ammo, just like an AK or SKS, 2 1/2 - 3". In my mind the only reason to have one is to shoot the military round for economies sake, not so much now, but I stocked up on the stuff in the 90's and have more than enough, ( did I really say that? ).
I recently bought a spanish mauser in 762x39, really cheap and nice looking, but so far with milsurp ammo, no difference, although I will play with some cast heavy bullets in it for fun, I have not cast the chamber and rifling yet to see if it is a .308 or .311 bore.
The Ak or Sks is probably a better bet, if you have access to inexpensive ammo. If you want to buy the good stuff, then why not step up to a 6.5 or 6.8.