dead space.

[Intel Xeon]

If you happened to use excessive twists and turns after the valve, before the barrel, then would the friction of the gas be enough to ignite the fuel mixture in a propane pneumatic?

I'm not sure if this has been carried out successfully or not but it might work.

EDIT: a heavy duty or metal piston valve seems plausible

[benstern]

propane compressed pneumatics don't ignite the fuel. they are only pneumatic. you are talking about combustion.

[paaiyan]

Even if you were talking about a pressurized fuel:air mix, which would be a hybrid, no the friction would not be nearly enough to ignite it. zotherwise natural gas pipelines wouldn't exist.

[mega_swordman]

No, it won't. The fuel to air ratio would not be in range (unless you have a couple yards of bends), and even if it was, friction doesn't give off enough heat for combustion to occur. The only problem with the cannon you are suggesting is that it would be weak with all the dead space.

[Intel Xeon]

Ok, thanks for the information!