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. The shell includes at least one front shell portion and at least one rear shell portion that are connected by a hinge so the rear shell portion may pivot about the hinge with respect to the front portion. By operating a locking bolt on the rear shell portion, a user can selectively lock the orientation of the rear portion with respect to the underlying marker. The locking bolt may be spring biased to a locked position.