I have a 20 gauge fusil, and only have that because it was very inexpensive when I bought it used. If I was going to choose one now I'd think about one in 28 gauge as I already have a .54 rifle.
I think if you are going to shoot birds then you do need a smooth bore barrel for a fowler or fusil, not for a smooth rifle. Since you already shoot a 28 gauge then you know some of the limits of that gauge on birds. You do, however, have a choked barrel gun, and your muzzleloader will work a little less efficiently than your 28 gauge with "skeet" chokes. So if you can try a modern 28 with skeet chokes and target loads, see if you like how well that hits on clay birds. If you really don't like it in the modern version, then you probably will want to send the muzzleloader barrel out and get it "jug" choked, which should get you to about "improved cylinder" shot patterns or perhaps tighter. Jug choking is the old style and still allows you to shoot round ball for deer.
As for multiple projectiles mentioned above, I'd caution the use of buckshot, for in many states buckshot is outlawed for use on game. A jug choked barrel will give a large amount of versatility..., though beware for once you get the hang of hunting with BP, especially birds..., you're likely to let all your other guns gather dust. :wink:
LD