While I'm here, I'll make the point that combustions do not work on "explosions", although a lot of people seem to labour under the misconception that they do.
Folks seem to be confusing "explosion", "combustion" and "detonation".Well, for one, there are no explosive alkanes, they're all quite stable.
Propane fueled spudguns most definitely involve an explosion. Modern rifles using modern powders also involve explosions. A running automobile engine involves explosions.
Any rapid combustion is characterized as an explosion. Any rapid depressurization is also classified as an explosion.
Explosions are subdivided into two categories deflagration (rapid burning but burn speed is less than the speed of sound) and detonation (rapid burning with burn speed greater than the speed of sound). Essentially all modern guns, from a propane fueled spudgun to an M-16 are powered by deflagration explosions.
Explosives that detonate are not generally used in guns, homemade or otherwise.
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The peak pressure for propane + air (at 1 atmosphere) in a close chamber is 120 PSIG or less. Worse case scenario with a basic combustion spudgun is that the ammo jams in the barrel. The gun will experience a short pressure spike to 100 ~ 120 PSIG. The pressure does not last very long because of heat transfer to the chamber wall. Within a second or so the pressure in the jammed gun will be back near atmospheric pressure.
The amount of heat transfered to the gun is miniscule. A PVC gun fired once with a jammed barrel will only warm the PVC up by a few degrees C.
Many (most?) pneumatic guns are regularly pressurized to 100 ~ 120 PSIG, about what the peak pressure would be in a jamed combustion gun. As others have pointed out, the pressure in the combustion gun last for only an instant. In a pneumatic, the gun must withstand the pressure for a much longer time.