Kartographer suggests an AK74 and there is nothing wrong with that choice. But I prefer western firearms. The AR-15 is a great choice and the direct competitor with the AK74. Ammo is a bit more expensive but it is much more widely available. I can even buy it at the local Wal-Mart.
If you want a lot more stopping power, the go for the ever classic M1A (a civilian version of the M14) or the Free World’s Right Arm (FWRA) aka the FAL. Both shoot very common .308 Winchester ammo and are very good rifles.
If you have those stupid magazine restrictions in your state, then get the greatest battle implement ever devised; the M1 Garand. They are a great buy and you can get a service grade (functional with a good barrel but definitely used) for a mere $595. The 8 round clip is legal in all states and with a little practice you can easily put 30-40 aimed shots downrange.