calculate recoil for big bore revolvers?

This will give you a general idea:

http://www.chuckhawks.com/handgun_recoil_table.htm

This will get you spot on:

http://www.handloads.com/calc/recoil.asp

1500 grains is 4.67 bore. There were some nominal 4 bores used in the late 19th century for African hunting, but most of them were really 5 to 6 bore, so the gun you ask about is a huge one, if it really exists. Black powder can't achieve 2300 feet/second with such heavy bullets. I read about some of these big rifles. One was an actual 5.6 bore with 1250 grain round lead ball or 1750 grain lead conical bullet. I forget the exact velocity, but it was lower than what you list, I think about 1500 ft./sec. The rifle weighed 18 pounds and had 210 foot-pounds of recoil. You're proposing a lighter rifle with a heavier bullet at far higher velocity, so I guess the recoil would be 250 to 300 foot-pounds. I doubt your figures for such a gun. Where did you find it? I know a man who has a .460 Weatherby rifle. With handloads, it fires a 500 grain bullet at 2700 ft./sec., and the rifle weighs 12 pounds and has 120 ft.-lbs. of kick.