Assault weapons are the primary (firing) weapons designed to be used in an (infantry) assault. Weird, huh? A few properties follow:

  • Easily portable. No, your favourite HMG, which requires a three man team to carry, cannot be the primary weapon on the assault.
  • Mid-range effectiveness. If it's long-range, it isn't supposed to be the assault people's job to kill it. In particular, pistols, mortars, commando-style knives and ICBMs are all not assault weapons.
  • Semiautomatic capability, and usually also automatic capability. It's bad enough you have to reload, you don't want to have to start peforming arcane rituals with various levers on your gun.
  • Standard ammunition. If you're NATO, it's going to be a 5.56mm (.22 inch) caliber weapon. If you're Warsaw pact, it's a 7.62mm weapon. Of course, these calibers were decided on the basis of some assault weapon, so you could switch the caliber while switching your assault weapons. But you always want a lot of ammunition for your primary weapon.
  • Weight. Nearly every one of your soldiers is going to be lugging one around. This includes weight of magazines.
  • Large magazines. Of course, this contradicts "weight" above.

It's a military designation, not a gun control one. Some people actually believe guns belong in the army. That's why it's called an assault weapon, not a GBH weapon.