155. Ground Signal Projector M3 and M4, resemble small mortars in that they consist of open-end tubes, with closing caps at the opposite ends that hold a firing pin which igni-tes the primer of the ground signal when the mortar is struck against the ground. The signal is inverted and loaded into the muzzle of the projector. Leaving the projector tail first, the signal reverses itself after attaining about 100 feet altitude. It then proceeds nose first until it reaches an altitude of 600 feet, where the expelling charge ejects and ignites the contents of the signal body.