Colt Defense M203 Grenade Launcher, caliber 40mm grenade. I am a grenade launcher commonly found on service rifles that are fitted for the Colt AR-15 platform. You'll always find me at the very bottom of the barrel. Oh right, manners. Launcher, Grenade, 40mm, M203, reporting for service duties, probably for the last time. USAF said that this year's the last year that older M203s will be replaced. It's been a good run.