After re-reading much of this, I'm positive that Command Car's suggestion is spot on - slowing them down might being them together, perfectly. Reduction in charge is replaced with light weight wadding to maintain the slightly compressed load, so the powder burns correctly.

At the original gun was a BP gun and it would have been regulated that way, slowing it down to around 1,800fps should bring the barrels together. I think you might even have the corect ball size. I'd play with those a bit more, first, as you are so close to a correct solution.
Buying expensive moulds at this time, would not be the best use of your spare change. Real BP and lead would be. HA!