High-Low System in the context of M79 grenade launcher

The M79 grenade launcher is a single-shot, shoulder-fired, break-action grenade launcher that fires a 40×46mm grenade, which uses what the US Army calls the High-Low Propulsion System to keep recoil forces low, and first appeared during the Vietnam War. Its distinctive report has earned it colorful nicknames such as "Thumper," "Thump-Gun," "Bloop Tube," "Big Ed," "Elephant Gun," and "Blooper" among American soldiers as well as "Can Cannon" in reference to the grenade size; Australian units referred to it as the "Wombat Gun", while British soldiers adopted the "Elephant Gun" moniker. The M79 can fire a wide variety of 40 mm rounds including explosive, anti-personnel, smoke, buckshot, flechette (pointed steel projectiles with a vaned tail for stable flight), and illumination. While largely replaced by the M203, the M79 has remained in service in many units worldwide in niche roles.