Seems slow, to me. Shooting that low, be careful of the chronograph.

I'd expect 136gr. of powder to shoot faster, around 1,250fps.
The reason I feel it's slow, as my bro's 10 bore Bess broke 1,500fps with 100gr. GOEX 2F and a patched round ball. Mike could be right, about the slugging up of the ball.

Sounds as if it needs more powder, to me.

Wonder what Cpt. Curl gets in his 10 bore?

I was just thinking about my 12 bore, and it took 190gr. 2F with the undersized 480gr. ball in a cup wad, to make 1,550fps
buy only 165gr. to give that vel. with the same ball cloth patched, in my 14 bore rifle.