ArmyProperty.com

The M320, a 40mm grenade launcher, is under development and intended to
replace selected M203 series of grenade launchers on M16 rifles and M4 carbines.
Interoperable, it attaches under the barrel, forward of the magazine, and also converts
to a stand-alone weapon.
The integral day/night sighting system, unrestricted breech mechanism, and modern
firing trigger deliver superior overmatch capabilities.
Located on the side of the launcher, the new sighting system design lessens
interference with rifle and carbine sights, reduces attaching operations to one action,
and eliminates the need to re-zero after reattaching to a weapon.
Opening to the side, the unrestricted breech mechanism is capable of firing all
explosive, smoke, and illuminating grenades fired by the M203, as well as a variety of
newer, longer rounds and some non-lethal ones.
In front of the trigger, the new pistol grip design eliminates the need to use the
magazine as a hand grip. The M320 functions in double action mode with an
ambidextrous safety. If the weapon misfires, the operator can pull the trigger again,
compared to the M203, which requires cycling of the breech to re-cock the firing pin
and pulling the trigger again.
As with all developmental weapons, the latest in lightweight material composites are
used to lighten the Soldier’s load and improve durability.
Source: http://peosoldier.army.mil/factsheets/SW_IW_M320.pdf