
1. How can the flame from the combustion go through two ball valves (on the propane meter), the tiny hole of the regulator and the tank's pin valve?
2. Even if it would, it wouldn't explode since there's no oxidizer in the tank, only fuel. We've been through this a dozen times.
And on the carbide cannon issue: BLB is right. I've seen ghetto carbide cannons made from tin cans or drain pipe that worked just fine. I've even ignited acetylene in a PET bottle and it didn't even bend it. There's just not enough time for heat to do any damage, 'specially since acetylene burns faster than propane.
However, since the pressure peak is higher and DTD is more likely to occur, I would only recommend it in short metal chambers, NOT PVC.