I was intrigued by this, so I did a little research.
Apparently, he states that he HAS built a prototype, but that it was destroyed due to the tremendous torque.
And, in reading through some statements by him, I noticed one GLARING misunderstanding by him:
That he is worried we will 'use up' all the hydrogen in the world like we are doing for fossil fuels, of course failing to understand the basic tenet that hydrogen exists in many states, natural and otherwise, and we could not ever conceivably ever 'run out' of hydrogen in the known universe. Also, he fails to understand that hydrogen is an energy carrier, not an energy source.
It looks like he maintains a 'wiki' page, and has contact info in case anyone is interested...
See these links:
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PESN Gun Engine InterviewGun Engine Explodes Fuel Efficiency
Bon Kulman interviews Kazimierz Holubowicz, inventor of the gun engine, which was designed with the objective of increasing engine efficiency from 20% to much more than that.
by Bon Kulman
for Pure Energy Systems News