The Manmelter is similar to another one of the Pyro's weapons, the Flaregun; it shoots a burning projectile that sets enemies on fire.
Being one of the Dr. Grordbort's weapons, it does not require ammunition and does do random critical hits. The Manmelter's projectiles also travel 50% faster than the standard Flaregun and cannot be deflected. The final ability the Manmelter possesses is the power to extinguish burning teammates with alt-fire. By doing this, the player is able to bank guaranteed critical hits in a manner similar to the Engineer's Frontier Justice.
This weapon is part of the Dr. Grordbort's Moonman Pack set for the Pyro. However, there is no set bonus for wearing all of the pieces of the set at the same time.