The M78 consists of two parts: the launcher and its power pack, which together weigh 15.2 kg. The gun itself is a 15 mW phased-plasma system that fires vaporized cadmium telluride pellets from a 30-round feed. The fast-discharge system powering the gun's laser and magnetic coil is fed from a 4 mW hydrogen fuel cell in the power pack. The firing cycle takes 3 seconds, most of which time is taken up by the powering up of the discharge generator.
The weapon's penetrating power is considerable, and it is claimed to be able to breach the flank armor of a heavy tank at 1,000m. However, many Marines show disdain for the weapon's considerable weight, and prefer the more conventional M5 RPG despite its reduced anti-armor capabilities.