2-p, my guess would be that the exact shape of the initial combustion chamber would be important and the exact fit of the wad against the hull wall. If the wad had a somewhat loose fit, there could be a bit of initial blow-by. If the wad had a very tight force fit to the hull wall, there could be some initial resistance to movement above the wad-shot weight. We know that peak chamber pressure is pretty sensitive, so these factors seem plausible to me.