I suggest a .308. You can pick up surplus military ammo for the coyotes. Accurate enough to kill them, and cheap. You can get suplus military ammo CHEAP. I used to shoot portuguese surplus that shot 1 in groups fairly consistant at 100yds. Rarly tighter than that, but accurat enough for yotes and practice. Youcan buy better ammo for your deer needs.

I strongly suggest you stay away from the 7.62x39 for anything put plinking. It's cheap ammo. That is about all that is good about it.