Shocking gas gun experiments

This is a detailed explanation of the gas-gun experiments which led to the SHARP project headed by John Hunter.

I wanted to see if such a device could be used for space propulsion.

It has occurred to me that the rocket engine is very wasteful of energy.  If a space-gun could be substituted for a rocket engine, much less energy would be needed to accelerate a mass in space, or so I speculate.  Since much less energy is needed, much less mass is needed.  Mass is the big enemy in space.  If much less mass is needed, less propellant is needed and so forth.