Select-fire assault rifle designed by a Russian guy named Kalashnikov. Semiautomatic-only versions are legal to own in most American states. Ruggedness and reliability are excellent. AKs basically work the way they're supposed to every time and are nearly indestructible, but the accuracy is nothing to write home about. (Cf. AR-15, FAL)
I accidentally ran over my AK with my truck and it still worked!
Semiautomatic version of the M-16 used by the United States military, legal for civilian ownership in most states thereof. Uses the .223/5.56 NATO round. Like the M-16, it's accurate but not very reliable because of its direct gas impingement system.
My buddy got killed by home invaders because his AR-15 jammed. He should have bought an AK or FAL.