Antique pre 1939 muzzle loading shotguns can be held off licence as long as you are not going to shoot them. Owning black powder or percussion caps for any reason indicates that you are. If you wish to shoot them they they are put on your SGC not FAC The percussion caps are perfectly legal to own and can be bought on a SGC but cant be posted.
Hope this helps.
I am not really sure about antique muzzle loading rifles.