Here's A Nitro load I shoot in my 1881 Scott 10 ga hammer gun...the gun was reproofed at 4 tons with 3 & 1/2" chambers.
Federal 10 ga. 3 & 1/2" hull, Win.209 primer BPD wad with 20 ga. Fiber fillers 1 & 5/8 oz of Bismuth 4 shot, 33.6 grains of IMR 7625, 1260 fps. I shot this at a measured 40 yards.