God, this thread just refuses to die... I'll repeat:
1) Uzi has a static firing pin, and the bolt is held back in cocked position by a sear. When sear releases bolt, spring pushes it forward, round gets fed, and as it enters chamber, static firing pin strikes the primer, igniting it - boom. However, a strong jolt can disengage bolt from sear and cause an AD - accidental discharge. This applies to any and all weapons that operate from open bolt - most SMGs, MAG, etc. Interesting side effect is that it prevents cookoff - a round only enters chamber when you are firing. M16 operates from closed bolt, so an AD of this fashion cannot happen to it, although it can empty a whole magazine from cookoff as LoTekK described - round enters chamber, detonates from heat, bolt is driven back, feeds another round, repeat until magazine empties.
2) You want a grenade, use a grenade. Uzi, as I would like to point out, is a submachinegun.