7.62x39 is a 100 yard hunting round at most. I've taken a couple of our small Texas whitetails with a Mini-30, and both were one shot kills. Bullet placement is the key.
7.62x54R is about equivelent to a .30/06 military load, with 150 grain bullets. Smaller case gives up some with heavier bullets, but it is still a very effective hunting cartridge. I use a Mosin Nagant M91/30 "homemade" sniper as a bad weather hunting rifle, and my oldest son prefers to hunt with it. We have taken several whitetails with it.