AR is fine for plinking, though ammo is still expensive. AK fine for hogs, but the specs on it are too lose for accurate shooting past 100 yards. I got a Rock River LAR 47 in .762-39 and it rocks. I wouldn't feed it that Wolf, Tula, etc or other cheap Russian ammo, because the specs are tight, and it can jam.
The really sick cartridge is the 6.8 SPC. You can get it in the Remington ACR, Daniel Defense, etc., but the ammo is expensive and hard to find (like every other round, I guess). The ballistics on that round are insane--more energy at 600 yards than the 308 Win, can adapt to many NATO .556 weapons, including magazine in some instances, and uses the Rem 300 cartridge necked down to a 270 Win bullet.
Flat, with excellent maintained terminal energy. For some reason even the armorers can't fully explain, barrel length does not seem to significantly affect muzzle velocity. The Army will likely replace the .556 with this, so soldiers won't dump their weapons for the much heavier .308. True long and close range capabilities.
Remington was supposed to mass produced several hundred million cartridges this year to flood the market and get the price down so people would switch. Like ever other cartridge though, it remains expensive and scarce.
I'm sure the IRS and Postal Service have tons of it.