Cannonmn,
I have seen very little written about these, other than some passages on the Spanish American war, Bannerman mentioning displaying one and Teddy seeing it,
during the return parade for the troops,
By your description it is a two chambered piece where by the lower chamber houses the propulsion charge and transmits the impulse somehow to the upper chamber,
I was under the mistaken impression that these were in effect Pneumatic cannon, or did the black powder charge compress a piston that gave the upper barrel a forced
shot of air?