keg,
From the sound of the guy I don't think that he would have a problem shooting the 120 with a full load of black.
I used around 115 grains of 1F with I think was a 550 gr GG or a 400 gr flat nose and they played down Michigan UP bucks in fine shape. Recoil is stiff but manageable shooting game off hand and with the right bullet shape he will shoot several follow up shots with out fouling control.
Heck I'm 82 and still shoot my Shiloh .50-2.5 with a 700 gr bullet or the .44-90 bn with a full load of black.
I don't think Doc has any black powder left. It sounds like the gun has been in the closet for a number of years. He does have some 4064. He also has a 50-90 as I recall but he never mentioned bringing it out.
kwg
