They were built way back, I think even before the flint - to the matchlock? I seem to recall seeing a photo of a superimposed matchlock- but can't recall where.
: I don't expect the charges were very high, and the closeness of the locks sort of shows that. Actually, the charges seem to be fairly long, given a calibre of .45 or .50. Perhaps there was an extra wad on TOP the ball behind the powder of the load in front it. It would be extra insurance against chain disharge, and take up some of that space as well. I am thinking you'd want the vent just into the base of the charge, to allow for some wadding up action from pressure in front, expecially for the second charge in the barrel. I expect by the time the first charge loaded or last to fire was let off, that paricular ball would be a fairly snug fit.