Cadet's reason for the "Vented" percussions is also the one I have always heard. Some folks thought they needed to get Air for better burning. Of course the powder had its own built in oxygen supply, it could not have received enough through that little vent without it.
To answer the original question we really have to keep all other parts of the equation on an even basis. In my opinion we need to stick with ML'ing caplocks versus breech loading blackpowder shells with equal loads of powder & shot. Using smokeless in the shells would add a far different variable.
Black has the unique feature of burning at essentially the same rate regardless of outside influences. It would thus be far less sensitive to minor differences in Crimp Strength etc as would smokeless.
The original question though only asked about the two systems. "IF" the question was which has the higher pressure a black powder ML'er or a Smokeless powder breech loader the Breech loader will win for certain.
Gotta compare apples to apples, not apples to oranges.
