Once they’d all found places to stand, T.R. said, "This here upper part will be filled with hydrogen gas, which is the working fluid that actually pushes the shell up the barrel. The physics of it is beyond me, but they say that the maximum velocity of the projectile can’t be any faster than the speed of sound in the gas that is doing the pushing. The speed of sound in light gases like hydrogen and helium is higher than that in air. Enough to make a difference for our purposes. We looked at helium. The world’s helium supply actually comes from up Amarillo way. It’s safer, but it’s expensive and hard to work with. If you build one of these at the doorstep of Germany, Your Majesty, you’ll want to take a hard look at helium. I can help you get some."