Muzzle-fired
Many grenades have been designed to launch from a rifle's muzzle, usually using either a special blank propellant cartridge, or more modern "bullet trap" and "shoot through" types which allow the grenade to be fired using live rounds. This system has two key advantages: the grenade can generally be made larger and more powerful as compared to underbarrel or standalone weapons, and the rifle's weight and handling characteristics are not affected as with underbarrel systems.
The disadvantage of this method is that when a soldier wants to launch a grenade, he must mount the grenade to the muzzle prior to each shot. If he is surprised by a close-range threat while preparing to fire the grenade, he has to reverse the procedure before he can respond with rifle fire. Rifle grenades also tend to be more difficult to fire accurately compared to under-barrel or standalone designs.
The SIMON breach grenade is a muzzle-fired grenade for breaching doors. The SIMON launches using a bullet trap to capture a standard 5.56 bullet fired from an M4 carbine or M16.
Read more about this topic: Grenade Launcher