[kronckew]

as old as these appear, the 'extra' holes may have been added well after they were no longer used. they are obviously not mounted as they would have been during service, as they are now just propped up on blocks. probably would have had a bit finer surface finish way back then as well. i've seen a sketch of an early breech loader where the breech block was held by a pin thru the centre of the breech with the touch hole forward of the pin hole. the breech block did not contain the charge, just sealed the end. most designs used a wood wedge to tightly force the plug against the breech, and thus has a square section behind the cartridge/breech plug to hold the wedge.

in any case i would likely have preferred being the target than the gunner with that frangible beasty. at least i'd want to be far away when it went off.

only somewhat related, i remember a episode of 'myth busters' where they made a cannon entirely of duck tape that fired an 18 pound ball rather effectively. held up for a few shots if i recall...