between elements 36 and 12 venting ports 24 and 34 on cylinders 16 and 12 respectively are provided for reasons discussed below in addition a venting port 40 is provided on cylinder 12 closely adjoining base 15 and covered by piston flange 30 when the launcher assembly is in a static state shown in fig1 at the start of the launching sequence when it is desired to launch mass 20 a blank round in the breech not shown of barrel 14 is fired gases from the fired round then expand as much as possible filling barrel 14 and exiting through the muzzle thereof as suggested in fig1 pressure is thus applied by the gases to the closed end 22 of cylinder 16 rapidly moving the cylinder and components affixed thereto forwardly with a fast rising acceleration when sufficient forward movement of cylinder 16 has occurred for port 24 to move forwardly of seal 26 flange 30 will be at or beyond port 34 as seen in fig3 where the flange is substantially aligned with the port effectively blocking the same in this operative state gases from the muzzle of barrel 14 escape through port 24 into the annular chamber 28 defined by the space between cylinders 12 and 16 as limited by flange 30 at one end and seal 32 at the other end of space 28 continued forward movement of cylinder 16 as shown in fig4 for example causes vent port 24 to move under seal 32 effectively isolating the entrapped gases in space 28 with no path of escape the gases thus entrapped are under substantial pressure and strongly resist any further forward movement of cylinder.