The M203 grenade launcher entered service with the U.S. military in 1969 during Vietnam. It replaced the M79 “Blooper” stand-alone launcher, almost always being used as an under-barrel addition to an assault rifle.
Though it has served faithfully and effectively for over 40 years now and will continue to do so for years to come, the M203 is being phased out of Army service and is being replaced by the new M320 designed and built by Heckler & Koch.
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http://www.businessinsider.com/us-military-m320-grenade-launcher-2017-1