Tinker found the same thing - has to do with the recoil increasing more quickly than the speed - I expect.

What was strange, though, was that my 12 bore smooth double, shot both tubes together at 100yards with both 7 dram load of 2F GOEX = 190gr. producing 1,550fps. The smokeless load of SR4756 that kicked about 1/2 as much (felt less than that, even) but produced the same 1,550fps speed with the same ball and also struck ON at 100yards - to the sights, the same as the BP load. 150gr. black powder and ball shot well at 50yards and was the intermediate load listed in Greener's book. I did not try the light load he listed as that produced less speed than I wanted.

Maybe I was just lucky with that gun. The tubes were undersized at .724" and were originally choked IMP MOD = .702"

I shortened them to 24" from 30", which beefed up the wall tickness considerably, and increased the distance between the bore centres (reduced the convergence of the muzzles).