Combat experience with the bazooka rocket launcher in World War Two and its larger versions in the Korean War convinced the US military that a better weapon was needed to give front-line troops a direct-fire way to attach enemy strong points. The bazooka was bulky, not particularly accurate, and created a lot of backlist signature when fired. This led to a multi-part development effort involving design of a small grenade body, reliable but cheap fusing system, and a cartridge design that could launch it.
The result was the 40x46mm grenade. It uses a “high-low” system (originally developed by Rheinmetall during World War Two) in which a powder charge is fired in a small compartment within the cartridge case. The initial pressure in this compartment is some 35,000 psi, which is plenty high to ensure complete and repeatable powder burn. At peak pressure, the internal compartment ruptures, allowing the propellant gasses to expand into the full case volume, which lowers the pressure to about 3,000 psi. This lower pressure is safe to use with an aluminum barrel, and propels the grenade at about 250 fps, giving it a range of about 400 yards without generating excessive recoil.
The M79 proved to be very accurate and reliable. It’s downside was the need for a grenadier to carry a backup sidearm, as the M79 could not be used at close range. Almost as soon as it was introduced, work began on developing a launcher which could be attached to the M16 service rifle. This would first be the XM-148, and then ultimately the M203 that would replace the M79 in service. M79 launchers can still be found all over the world, however, as they are robust and reliable.