BP load producing identical speeds will kick more.

If you have a 12 bore, you can prove it to yourself - SS break open would be best - as to felt recoil- I personally would not shoot the BP load from a SS.

Load a 2 3/4" case with 7 drams - 190gr. of 2F black powder + a .724" or so, round ball.

If it's a SXS - the recoil may split the stock - fair warning.

1st shot from my side by side - surprised me - spun me 1/4 circle- almost 1/2. I had been shooting smokeless loads - not even close. Had to repair the stock due to split tang. Then, it was OK, but sure preferred the smokeless after that.

Load up a 2 3/4" AA case with 44.0gr. Blue Dot and a 525gr. slug with the same round ball.

Same velocity, 1,500fps but 50% less felt recoil.

The recoil formulas do not work with black powder.

Too - I can easily fire 1,600fps loads with a 506gr. G/C bullet in my Sharps using 42.0gr. of Benchmark, yet 122gr. 2F, producing 1,500fps kicks considerably more.

You could also compare factory .45/70 ammo with 405's at about 1,320fps and a full compressed load of 65gr. 3F with 405 jacketed or cast. The BP load might make 1,230fps yet will kick more.

56% by weight of black powder results in solid waste when burnt - ejecta - added to bullet weight.