A gun or other projectile launching device includes a frame, and a barrel, including a bore therethrough, held in position by the frame. A cylinder is rotatable attached to the frame in close proximity to an inner end portion of the barrel, the cylinder having multiple chambers and configured to sequentially and longitudinally align a chamber with the bore of the barrel. A sleeve is slidingly positioned over the inner end portion of the barrel. The sleeve has a front face with a central opening therethrough, that has a size less than an inner diameter of the sleeve, but no smaller than a diameter of the bore through the barrel. The sleeve is configured to slide back, substantially in response to gas pressure, until the front face of the sleeve is forcibly maintained in contact with the cylinder to eliminate a gap between the cylinder and the barrel.