Assuming you are fueling with propane+air at 1x and it is a true mortar (fairly short barrel relative to the pipe diameter), I would put the hazard rating at a 2 or 3.
If you can actually get the fire rate up to one shot every 5 seconds then the safety rating might go up to a 7 or 8. But I would worry about pressure rated PVC at that high a fire rate as well.
Cellcore PVC works fine for any combustion gun using propane+air at 1x.
