I don't know what you are putting in a 20 gauge shotgun shell of modern design, but black powder does not detonate as does smokeless, so any recoil is more a shove, than a sharp slap to the shoulder and face. All recoil is felt according to the design of the stock, the weight of the gun, and how the shooter mounts the gun to his shoulder and face. If you screw up any of these factors, you can be hurt, although usually not badly.
Since you are new to the forum, I will hazard you are new to Black Powder. I therefore commend to your reading Bob Spenser's Excellent website, Black Powder Notebook. He has two articles on loading shotguns, one he wrote, and the other from an old master named V.M. Starr. I recommend that you read both articles before you consider the first load you put in any shotgun.
http://members.aye.net/~bspen/index.html
You should also search out and buy a copy of Lyman's Black Powder Manual, second Edition, by Sam Fadala. Its has loading information for most black powder guns with down range ballistics to satisfy your curiousity.
Black Powder shotgun loads are rarely loaded to a level above the speed of sound( 1135 fps) Starting below the speed of sound gives the pellets the advantage of not having to come down through the transonic zone, where air turbulence buffets them around quite a lot. That allows black powder loads to give reasonable patterns out of cylinder bore barrels at 25-30 yards. Black powder barrels can be " jug " choked, to let you have your 40-45 yard gun again, but you are not going to get the same high velocities using BP as you did with your smokeless powder charges. Its when people try to get that same velocity that recoil levels rise precipitously, and they get hurt.
The secret to killing birds at the 30 yard range, or further, assuming a choked barrel, is to use larger size shot than you would normally expect to use for the same game if you hunted with Smokeless Powder modern shells. If you hunt dove using #8 or #9 shot with your modern gun, consider using #7 1/2 or even #6 shot with your BP shotgun. If you kill pheasants and grouse with #6 shot, use #5 shot in your BP loads. Retained pellet energy is what kills birds at any distance, and the larger shot carry energy better, no matter how fast they leave the muzzle.
Oh, and even if you use Smokeless Powder, all that velocity that causes the slap to the shoulder is lost in the first 20 yards, according to Lyman's Shotshell Reloading manual. You don't need that extra velocity out to 20 yards to stop anything, because your patterns are going to be so dense at that short range. So, you may even learn to back down those 20 ga. loads you now use in your modern gun.
Yes, you have to pick your shots, and pass on longer shots, but you probably don't have a huge percentage of kills on birds out at 40 and more yards, anyway, if you honestly consider all the shots you have taken at birds at that range. Most of us don't, either.