Assuming this is a serious question, underbarrel grenades are much quicker to deploy. The user can have a round in the chamber and the launcher on safe when using his weapon and switch immediately to his grenade launcher simply by raising the grenade sights, adjusting his grip, and taking the launcher off safe. A barrel mounted grenade requires the user to have specialty blank ammo, and he must first unload his rifle, mount the grenade, raise the sights, then load the blank in before firing.
Recently there has been a push towards switching to standalone grenade launchers, such as the Milkor MGL, which, although slower to deploy and bulky to carry around, frees up the underbarrel of the rifle for other accessories and allows the user to fire 6 shots, rather than a single shot, before reloading.
Ian Adams
Honestly they need to just switch all infantry to small diameter (probably 15mm to 20mm diameter) fragmentation grenade launchers with high capacity magazines in place of their standard issue rifles and carbines, maybe replace sidearms with sub-machine guns like the P90 and MP-7/Uzis if they need to aid the light machinegunner in suppressive fire or in environments where frag grenades are dangerous to the operators.
Basically they're great for that immediate reaction where you need shit blown up 'right now' and don't have time to fiddle fuck around fitting something on the end of the barrel. That being said, barrel launched grenades tend to have a bit higher charge in them which has its own merits, mechanically simple, low-no maintenance and they don't weigh the rifle down like an underbarrel launcher, that tends to add a fair bit of fat. Pros and cons to everything.
>how to turn a barrel into a melted banana >still fucking glorious anyway