Use will be the biggest factor in selecting a gauge. If you are a season long Waterfowl hunter, you want a DB shotgun, probably with chrome lined barrels, and designed to be able to shoot non-toxic shot. Screw in chokes are also a big help. Because you can't get the same range with black powder guns that you can with smokeless, you might want to consider a 10 gauge DB shotgun for waterfowl hunting.
If instead you are shooting upland game,but mostly birds, then you might want to choose a 12 gauge DB shotgun. Two shots allows you to go to town on high bag limit birds like Dove, or hunting at a Hunting Preserve, where you pay for what you take.
If you are interested in quail, grouse, and other small game, then there is no reason not to consider using a 20 gauge. A flint double is extraordinarily expense, because a good one is difficult to build. A percussion DB 20 will work well, but choose it only if you really prefer a lighter weight gun that what you get with a 12 ba.
Now, if you are into Rock Locks, ( as so many of us old guys are) then a 20 ga. fowler is the ticket. It can be used with RB to shoot deer, and with shot to hunt everything. You need special loads to hunt waterfowl legally, but everything else is fair game. You just have to pick your shots, and be a good wing shooter.
The 28 gauge is a terrific gauge, and is considered the " Small bore " gauge of choice for shotguns. It throws very nice patterns, the guns are very light weight, but the recoil is mild. However, its not the best choice for hunting anything beyond about 30 yds, simply because of the limitations on the loads to be shot. It would handle a .54 cal. round ball( .550 Cal. Bore ) and it would certainly kill deer. However, the weight of the guns tend to limit how much powder you can stuff into such a gun behind a RB and not beat yourself half to death hunting deer. Its no Hawken rifle, that often weight twice as much as these little shotguns.
So, decided what kind of hunting you want to do, then what action type you are most comfortable with. The animals you hunt, bag limits, and the kind of action you are likely to see in the field when hunting, will guide you in determining that one gun of a lifetime you want to purchase.
Then, if you are like the rest of us, once you get that gun, you will want another, and another. :rotf: :thumbsup: