The wikipedia article
suggests manual release.
It was very simple in that the shell closed the top of the chamber and was retained by a "gripper" until the air pressure was deemed sufficient and the gripper was manually released, which fired the weapon.
I can conceive of a shearable bolt that would fail at a predetermined pressure (as used by the PAW 600
, though that was in conjuntion with an explosive charge) or some sort of pop-off governed mechanism (pop-off actuates a piston which releases the projectile) though these would not result in an accurate firing time.
I'm curious about the studs on the projectile
, my first thought was that they might engage rifling in the barrel (similar to theLa Hitte system
) but they don't seem to project far enough.
I think the design in the patent that is the subject of this thread is a more elegant solution though, as the projecile is balanced and isn't under any strain (aside from air pressure, but that is also true of the mortar designs) until it is pushed forward for firing.
It seems the French were in on the act too, more details here