The term GUN, AS A ENGINE can be summarized as a device that uses the force of an explosive to propel a projectile. The crossword solution for this term is REV, which stands for revolution. A gun engine is a type of internal combustion engine that uses the force of an explosive to drive a piston or turbine, which in turn rotates a crankshaft to produce rotational energy. This rotational energy is then used to power a vehicle or other machinery.