Hi,
I’m reading "The Sporting Rifle and its projectiles." by Lieut. James Forsyth.(1867)
I enjoy the reading a lot.
On reading I read a paragraph very interesting as it is what I found when range testing my Kodiak:

“The only effect of increasing the charge was to lessen the elevation, and make the rifle hit harder; the recoil, of course, increased in proportion. It is of great importance that the sporting rifle should admit of varying charges without affecting its accuracy to any serious extent. This is the case with the spherical ball rifle as properly constructed, but with no other that I am acquainted with. The only effect increase of charge has on this rifle is to make the point-blank range somewhat longer, and to make the rifle hit harder; the accuracy remains unimpaired. If it be a double barrel, and it shoot true with the ordinary charge, an increase will have a slight effect on the shooting of the barrels, making each diverge slightly to Its own side. As such animals are always shot at short distances, this will hardly be perceptible; but some error of this description is unavoidable with double rifles used with varying charges.

This is exactly what I found.
I think that black powder increase recoil more than speed.
All double rifle manuals recommend to lower the charge in order to “uncross the barrels” but this must work only in smokeless loads or black powder very low loads.

Thanks
Martin