I know it sounds CoDish, but during the Vietnam war, US troops had grenade launchers attached to their SMG/Assault rifles.
The mechanics are that the GL`s take up your grenades. You can activate the GL by pressing G, the a targeting retucle for it appears as this:http://images3.wikia.nocookie.net/__cb57888/callofduty/images/9/90/Gl_4.png
It should drop a vast amount of blocks, 3 blocks for 1 and a half seconds. When in use cannot be aimed by sight. When you launch the mini grenade, it will cause alot of recoil, making you look up 20 degrees. You cannot shoot while running. You can only shoot when you are not moving, It has half the damage of a grenade, it makes a 2x2 wide hole. You get killed within 4 blocks of the grenade. Only SMG is able to have it. In the beginning of choosing weapon and you pick SMG, The game asks "Keep Grenades, Or Have GL(grenade launcher)". IMO, it will make people who use SMG look less nooby. It will balance out the weapons. Comment on what you think about this, could it be improved? Don`t like it? Would it be useful? Hope you read this idea.
-snipe