The M243 smoke grenade launcher is a 4-barreled 66 mm grenade launcher used on M113A2/A3 APCs and selected Humvees such as the Ground Mobility Vehicle.
The M243 primarily uses the L8A1/L8A3 round filled with red phosphorus. Upon activation, a dense cloud of white smoke is created from ground level up to a minimum height of and between from the vehicle in 2 to 6 seconds. The cloud lasts 1 to 3 minutes depending on wind speed and other weather conditions.
Unlike very similar grenade launchers (the M257 and M259), the M243 has a steel storage box mounted on the outside of a vehicle that stores four grenades.
See also Ground Mobility Vehicle – (US)SOCOM program - American Special Operations Humvees that can install M243s.
References
U.S. Vehicle Grenade Launchers - Gary's Combat Vehicle Reference Guide M243 (NSN 1040-01-059-0560), M257 (NSN 1040-01-070-1213), M259 (NSN 1040-01-107-7501) Smoke Grenade Launchers Technical Manual