I would try more powder. 100 gr is basically a shotgun load.
Rifles shooting a ball invariably need more powder unless twisted very fast.
Forsythe used 5 drams in a 16 bore rifle, this gives about 1600 from my FL sporting rifle with a 30" barrel.
This is about what Forsythe was getting.
I would try more powder, 30% of ball weight is not too much with a patched ROUND BALL of this caliber.
The problem with bullets with heavy recoil is the chance they will move off the powder when the first barrel is fired.
There were reason why bullets were not popular in MLs of this size. They could not be hardened and still expand and it does not take much to move a heavy bullet off the powder. Even carrying the rifle muzzle down may cause this.

Dan