Ruger 10/22, stainless, $275 on gunbroker.com, America's most basic little anti-rabbit/anti-possum device would require very little change to meet the military definition of an assault rifle. Again:
- Big enough to injure a human adversary badly enough to take him out of the battle.
- Otherwise fairly small, easy to load up with lots of ammo and carry lots of ammo around. That says small caliber; the tradeoff is that you assume you're not shooting anything past about 300 meters which is normal for most military situations. This eliminates the idea of taking 600 meter shots with an assault rifle. One guy in the squad carries something which CAN do that.
- Quick (aimed) follow on shots.
- Light, easy to carry and manipulate.
Add a Picatinny rail, a 20-round mag, and make it in a somewhat larger rimfire cartridge, 25 caliber, 85 grain bullet, 2500 fps, and you have your military definition of an assault rifle in spades.
One advantage of a rimfire/non-flared cartridge is that a 20-round mag would not interfere with prone position shooting.