The M79 was a simple single-shot, single-barrel, shoulder-fired weapon which broke open for loading the 40mm grenade into the breech just like a shotgun. Once loaded and closed, the firer put it to his shoulder, took aim through a simple open sight, and pulled the trigger. It fired a spherical grenade which, although just 40mm in diameter, nevertheless had a kill radius of five metres. Firing a large grenade from such a lightweight weapon presented some problems, but the ammunition design was such that the whole thing became very controllable and consistent. A rubber pad was fitted to the shoulder piece of the butt stock to absorb some of the shock.