Since Pyrodex needs higher pressures to burn well, maybe those old necked chambers are what it needs.
Now we're thinking through the issues!
I think we're in agreement that the pseudo-blackpowders need something to push to get to the point that they will/can build pressure.
In one of the REFERENCES on hand gonnes (posted in the Reference Sticky years ago) I read that the longer and thinner powder chamber (compared to short larger diamter) produced higher pressures.
My concept here is to build a gonne/cannon/howizer/mortar with a long thin powder chamber that just fits the pellets (or loose powder can be used) and to compare that with another very similar tube with a shorter but larger diameter chamber.
A candidate for this is (as DD calls it) the KISS design. We (and others) designed a two-piece barrel with powder chamber and tube that assembled to be the barrel. Easy enough to make a similar powder chamber (end) and compare using the same length tube and similar projo. Golf balls are good candidates as they are both very uniform and very light.