Larry Vickers’ TacTV YouTube channel has released a series of videos showcasing 40mm underbarrel grenade launchers, including uncommon footage of firing actual high explosive dual-purpose (HEDP) grenades. The videos are embedded below:
The underbarrel grenade launcher is one of the great innovations of the post-WWII world. While less elegant than rifle grenades, underbarrel grenade launchers can be mounted to anything that will physically accept them, and since they operate independently from the rifle, allow a lot more freedom both in the design of the rifle and the cartridges it fires.
Muzzle-fired rifle grenades are another similar concept that was developed earlier and saw use in the Second World War, however they suffer from some substantial limitations that the underbarrel launcher does not. First, they are highly dependent on the ammunition characteristics, both charge and projectile, of the rifle being used as a launcher. Whereas an M203 has exactly the same trajectory when mounted to anything from an MP5 to a G3, rifle grenades produce different trajectories depending on the rifle they are fired from.
Likewise, rifle grenades suffer from a dilemma regarding the projectiles used by the host rifles. The simplest solution is to use a blank projectile, but this prevents the rifle from being used as a grenade launcher, and then immediately again as a rifle (firing grenade, then “cleaning up” with small arms fire). “Soft” projectiles (e.g., M193, M80) can be used in bullet-trap and bullet-through style grenades, but armor-piercing projectiles are only compatible with the bullet-through type. Shaped-charge warheads, a very useful type that offers both an explosive radius, as well as excellent armor penetration capability even against well-armored AFVs, do not synergize well with the bullet-through configuration, meaning that essentially one has to choose between armor-piercing rifle ammunition and shaped-charge grenades.
Finally, rifle grenades negatively affect the life of the host rifle, usually cycling the action very violently while stressing the frame of the rifle much more severely than normal firing would. As a result, rifles optimized to fire muzzle-mounted grenades are typically much, much heavier and more robust than their counterparts.
The underbarrel grenade launcher circumvents all these disadvantages while providing additional accuracy of fire, with the tradeoffs of extra weight on the rifle (from the launcher itself) and smaller, less capable grenades. Many rifles, such as the M4, are compatible with both rifle grenades and underbarrel launchers.