My Mosin is my most fun "big gun" to shoot. A sealed military surplus "spam can" of ammo (440 rounds) costs about $79. Same as the rifle itself. You can't beat 'em for high powered economical shooting. A nice cushy slip-on recoil pad (like a "LimbSaver") makes the Mosin really sweet to shoot. Without a recoil pad, yeah, you will find you shoulder over in the next county behind you. The first time I picked mine up with the bayonet attached, I put a hole in the ceiling (a bayonet hole, not a bullet hole!) They are quite long rifles - at least the 1891/30 variety.
Even though it's done tongue-in-cheek, that website is dead on about the AK's and Mosins. I don't have an AR to comment on though ... my choice is three AK's and three Mosins for the same total cost as one AR.