I don't see any evidence of a pressure rise, or interruption of the fall in pressure, when the ejecta "slams"
into the forcing cone
You don't?!?! Since the ejecta will be entering the forcing cone before it's entirety has completely exited the hull, it's impact on pressure would be measured in the first 1-2 inches of the barrel, right where the highest pressure spike occurs.
It's not as if it is exiting the hull in it's entirety and traveling unrestricted through open air, and then "slamming" into a forcing cone as you put it.
While this discourse is interesting. The only way to prove this one way or the other would be to come up with an actual pressure test. One would have to come up with, as much as possible, two identical loads for both a muzzleloader and a breechloader and pressure test the guns. While it would extremely interesting, it doesn't really serve any purpose other than satisfying curiosity. Alas, I don't foresee Sherman Bell doing one of his fantastic test runs on this topic.
If I were a betting man, I would put my money on pressures being higher in the breechloader. As to what degree however, I could not say.
