That subject has been beaten to death on this forum. The general consensus is that there is no reason to use hydrogen over an alkane.
Utron would beg to differ...
Also potatos were not really my main projectile in mind I was thinking 20-30mm bearings, DYI
hence the need for licensing...
At 20X, it's obvious that potatoes weren't in mind. It's common practice around here to use steel/lead projectiles, and metal (or at the very least machined plastic) rounds are obviously required for any higher pressure projects. As you're in the US, it's not your projectile choice that hurts the legality of the project, it's the semi-auto fire capability. Were it a muzzle loader, it'd fall under the same category that muskets and old cannon do.
...because if piston seals fail and the reaction from the combustion chamber reach the meter pipe it will explode.
The meter should be disconnected completely or at least vented in any case, if only to protect upstream equipment in the event of seal failure. This is common practice for hybrids in general. If it needs to be done remotely, place a check valve between the meter and the gun, and a large solenoid valve on the meter. Simply open the solenoid valve on the meter after fueling, which vents it to atmosphere and prevents any dangerous pressure from building up in event of seal failure on the check valve.
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
