if you want a gun JUST FOR SHOOTING WATERFOWL, then consider the larger Gauge gun. It gives you more shot, to fill the patterns.
However, with any BP gun, you are limited in the yardage by the velocity you can achieve, and the choke in the barrel.
A Modern 10 gauge gun might be capable of shooting Non-toxic shot loads in patterns sufficient to take birds out at 50-60 yds. Your BP 10 ga. is going to be a 40 yd. gun at most, unless its choked.
If you are considering using that fowler or fusil for hunting deer, with a PRB, then go with the smaller gauge. A 20 Ga. RB will weigh about 320 grains, or 3/4 oz. and for the distance at which it is accurate( 70-80 yds.), its more than plenty to take any deer. The recoil from shooting heavier balls in the larger gauges becomes a factor in shooting them accurately.
If I were going to shoot a RB out of a 10 gauge fowler, I would choose Fg or, at most, FFg powder, and keep the load reasonable, based on the fact that the weight of the ball is going to give me a very good SHOVE when the gun fires, and I am putting into motion a substantial FREIGHT CAR that will pass completely through a deer if it hits one. High velocity is Not needed when your deer is 50 yds or closer to you.
That idea is hard to accept for new shooters to this sport, particularly if you have been shooting modern CF rifles until now.
If you are willing to limit the yardage at which you shoot at waterfowl( 30 yds or less), a cylinder bore 20 gauge with 1 1/4 oz. of shot will kill a lot of geese and ducks. :thumbsup: