If you're looking for an AR upper for hunting deer and so forth, why not get one chambered in 6.8SPC or 6.5 Grendel? Or even .300BLK? That is one of the reasons those cartridges exist, after all, and they are purpose designed to function in AR rifles. The 7.62x39 can have trouble cycling in an AR.
The 6.8 uses a standard bolt, standard magazines, and is very effective on game. It's like a .270 lite. A friend of mine has 2 deer, a javelina, and several foxes and coyotes taken with his 6.8 - 1 shot each. He has a nice little 1-4x Leupold on it. Can't imagine a better, lighter little walking around carbine. There is some momentum building behind this cartridge, as it is becoming a mainstream service and hunting tool.
The 6.5 needs its own bolt and perhaps magazines, but it gives somewhat flatter trajectory and uses the high ballistic coefficient 6.5mm bullets used in the 6.5x55 and .260 Remington. All three are very effective on game.
.300 BLK gives you basically a .30-30 in your AR, with better bullets.
Good luck with whatever you choose, but I would personally pick the 6.8SPC if I were you. With the mainstream and military ammo manufacturers getting behind it, it will be the least expensive modern hunting option for the non-handloader, but give better game-taking performance than any 5.45 could dream of. It has true hunting ammo loaded for it, which I haven't seen in 7.62x39 except for some questionable Wolf softpoints. Certainly haven't seen the variety of extremely effective modern hunting bullets you can get for the 6.8 in the old M43, anyway.
In fact, I will probably be putting together a 6.8 upper in the near future. It's a very practical package. Guys on the 6.8 forums are even taking elk with them.