Aren't far behind a modern shotgun with a BP double...the ballistics are only a bit lower, but with shotgunning, it's all about the pattern.
I actually prefer BP shotgunning, and will take out the percussion during any shotgunning season....just find it more enjoyable, and a slower paced hunt.
Do a search on black powder shotgunning...are some very good sites. Some advice:
1. Don't buy any wads until you have the gun in hand and can maeasure the bore. Many of them are 1/2 sizes...my 10ga. is in reality an 11ga. and needs 11ga. wads to run right. Some 12's are actually 13's, etc.
2. Plan of building your own loads the old fashoned way...card powder wad, felt wad (lubed) shot, and over shot wad. Plastic and BP aren't a great mix...but if you "just gotta" then make it: over powder card wad, lubed felt wad, plastic shot cup, over card wad.
3. If 13ga. or larger, do not be afraid to try Fg.
4. BP shotguns may be the dirties black powder guns. BP is naturally dirty...but considering the pressure shotguns run, it's even dirtier.