Lucky you to have a live firing piat. You can still get live piat cartridges, they are still about, collectors deactivate them. Not sure if they are filled to the top but don't exceed cartridge space of powder, the powder is most probably slow burning Balistite as found in 2" Mortar cartridges the Canadian equivalent is Hercules 81mm powder. In the 2" mortar cartridge a small black powder charge is used to help ignite the balistite, I believe it's difficult to fully burn with an ordinary shotgun primer, I doubt the piat cartridge uses this method. The cartridge itself is basically a rimless two piece 12bore cartridge with brass cap that stays in the round and the other part that holds the primer ejects out of the gun in front of the firing position, I believe this bit was dangerous to the operator. If you used a lighter projectile and a black powder filled empty piat cartridge as a measure I reckon it would work.. First I would check out the guys at Knob Creek in the US and see if anyone uses one there, I bet someone has one that they play with they have all sorts. If you do get to fire it post a video, that will be fun to watch.
