A convertible projectile launching device includes a projectile launching device, such as a paint ball marker, a less than lethal launcher or an airsoft gun, to which a user can secure at least one shell that changes the outer configuration or appearance of a projectile launching device on which it is secured into a MILSIM style projectile launching device. Any type of fasteners may be used to secure the shells to the device. The surface of at least one shell provides a rail system for mounting objects to the projectile launching device. The shell has a mating surface that engages a corresponding mating surface on the projectile launching device in order to increase stability of the shell. A blanking plug or a butt stock can be inserted into the rear of the shell, secured by a latch or other mechanism on at least one shell.