MK203 40mm grenade launcher, you typically see them slung under the barrel of an M16/M4. https://gunspot.s3.amazonaws.com/listing-photos/2020/03/06/IMG_5900.jpg Shoots one grenade then must be reloaded by hand. Unlike the tripod / vehicle mounted version MK19 (other countries have them and call them something else, but essentially the same item) https://vistapointe.net/images/mk-19-grenade-launcher-12.jpg