I sell one called the B&T Grenade laiuncher. It looks f-ing awesome.
I have another gun recommendation:
The Ruger 10/22
shoots a .22 round, cheap, easy to find, light.
Can legally and easily be fitted with a drum or double drum mag to hold 50-100 rounds (because it's a rim-fire semi-auto and not centre-fire)
Lightweight, quiet, very accurate. Perfect for small game, but also capable of a lethal head-shot up to 1400 yards.