ammo question

[Patrick Chadwick]

and 25 grains of some sort of filler like coarse sawdust

Maybe semolina? That is a filler often used by BP shooters. 85 gns fills the case up to the shoulder, and anything more is just painful for your shoulder Much less, and the filler could become compressed into a lump below the shoulder, in effect plugging the cartridge case. As muffet wrote, it sounds like a typical M-H load.