I have read a little about the risks of DDT on gaseous mixtures, but still i think the dangerousness of hydrogen is exaggerated in the mainstream knowledge, acetylene can suffer explosive polymerization and is even more prone to DDT but still is much less feared, in the writing i was learning about this they induced detonations even on methane-air mixtures, for this they used something like a plasma capillary with fast discharge times to induce a detonation in gas mixes within pipes.
I didn't know those existed, similar to what happened with 4500psi hand pumps
, so you are thinking in using the full 9000psi in your gun 
thats some mental pressure to contain especially if its to be deflagrated.