a 308/260/243/7mm08 and so on are better for any kind of long range work on mid to large sized game.
the 7.62x39 don't seem to gain a lot from longer barrels, a 16" is plenty for all the x39 has. good for mid-ranges shots on deer , and cheap to shoot.
the 6.5 grendel is a better at long range on mid sized game than the 7.62x39 and will fit in the ar-15 lower.
any of them can take a deer so it is a size,weight and expense trade off. pick the one that comes closest to whats needed.