You need to be familiar with gauges, and what that means in caliber. A 28 gauge gun is roughly .550" caliber. A 20 gauge gun is rounghly .620" caliber.
Also mentioned above are both the 16 gauge( .662" Cal.) The 12 gauge(.729" cal.) and the 24 gauge-.579" cal.)
The reason to not put a Patched Round Ball on top of the load of shot is that it is HARD on both the gunstock, and your shoulder, to shoot, with that much recoil. The combination also slows both loads, so that the PRB is not likely to strike the same Point of Impact as it will being shot Alone out of the same gun.
If you are going to also hunt Waterfowl, you need to consider a gauge/caliber that will allow you to throw enough shot pellets to give a killing pattern out to 25 yards. Here, that generally means using a 20 gauge or Larger shotgun, with the 20 gauge generally being considered marginal.
Remember that you are Not going to get the same kind of velocities using Black Powder in a shotgun( smoothbore) as you can using modern smokeless powder cartridges in breech loading shotguns. That means you need to increased PELLET ENERGY on Target( game) by increase the size of the shot you shoot. The larger pellets carry energy better.
If your hunting is going to involve more waterfowl shooting, than upland game and deer hunting, then consider a 16 gauge smoothbore. The RB weighs almost one ounce- a HUGE ball to hit any animal- including wild boar. The Diameter of the bore is wide enough to allow healthy loads of shot for Duck shooting. If you are allowed to use Lead Shot in Norway, still, for waterfowl,( non-toxic shot is required here in the U.S.A.), lead shot that is #6, #5, or #4 is idea for hunting ducks, and can be used for most all upland game, too.
Flintlock or percussion? My only suggestion is to try to find a club where members have both, and try them both out. Those of us who have crossed over to Flintlocks are very partial to those guns, but we would be the first to tell you that there is a learning curve to climb before you can get the guns to fire reliably, and to shoot them well. With percussion guns, which are a bit closer in mechanics to modern breechloaders, the learning curve is a bit less.
All muzzleloaders require some NEW THINKING compared to shooting modern cartridge rifles and shotguns. Luckily, you are here, and you can get any question you have answered here. We can help you with those learning curves.
The problem for anyone in Europe is local gun laws- and availability of Black powder, and percussion caps. (Gun)Flints are available from both England and France, from different suppliers, so that is no problem. Wads, cards, and cleaning patches can be obtained from here by Mail order, if they are not available locally. I can only assume that lead shot is also available in Norway, as you would have mentioned that as a problem before now. RB molds, to cast your own balls are available from Tanner, in England, or you can buy cast balls from several sources here.
On the index page to this forum, go up to the box marked "member resources". Then, scroll down to "Articles, Charts, and Links." If you click on Links, you eventually got to a list of suppliers. Click on their names, and you get their own websites, with catalogs, and contact information for ordering. While you are at that place on THIS site, check out the articles, and the Charts available to you. They are Tools to educate you, and to help you find out various USEFUL information.
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