The Boost Machinegun is a downloadable content weapon. The magazine is small, but since it has comparable ammunition reserve as the heavy machinegun, the boost machinegun is a good complement for the assault rifle and heavy machinegun. As your magazine runs dry, switch to another weapon and your volume and rate of fire remains constant.
The one quality about the weapon is that it can penetrate floors, walls, and obstacles up to a certain thickness around level 6 or 7 (stars). This means the Chicane (wall making robots) and other enemies who take cover can be shot to death if you power-up this weapon enough (and dying when the weapon is at max-level won't delete that ability since it needs to be de-powered a few levels to lose it).