A gas gun launcher having a pump tube and a launch tube with a first end of the launch tube slidably inserted into a second end of the pump tube. A sliding seal is employed to retain the gas within the launch tube and the pump tube A launch tube alignment system is preferably automatic, again to enhance the accuracy of launches. And an embodiment of the gas gun launcher suitable for use in water such as an ocean or large lake preferably utilizes a neutrally buoyant launch tube and a neutrally buoyant pump tube. And a fast-closing muffler at the second end of the launch tube conserves the light gas utilized for launching a vehicle.