Welcome to Modern Muzzleloader, Tom ! Good name!
Either true black powder or one of the subs like Pyrodex or T7 has likely been shot in the gun and either moisture from cleaning was left in the nipple area or the gun was not cleaned after its last use. In any case, you'll do best with hot water to clean it. On that rifle I'd remove the barrel from the stock and pull the bolt assembly and breech plug. Run a patch thru the barrel dry to make sure the barrel is in fact not blocked with crud, then clean it by submersing the muzzle end in an ice cream pail of very hot water Start a patched jag in the breech end and run the patch to the muzzle and then back without removing it past the rifling where the threads end for the breech plug. You can start a pumping action doing this which will wash the barrel clear. Dry the barrel with a couple clean patches and wipe down the exterior well. Check the barrel to be sure the rifling has cleaned up. If all is well, run a lightly oiled patch thru the barrel and set aside while you clean the breech plug and nipple. Give the bolt a look see to be sure any residue on it gets wiped off paying particular attention to the firing pin area. Since the gun hasn't been fired in those years, I'd disassemble the bolt and clean all the components well and reassemble after giving everything a wipe with a lightly oil cloth. Take a peek down the barrel and if it appears clean with crisp rifling you're good to go.
Round up what you need to shoot the gun and give it a go. Clean it right away when you get home. If the barrel was clean and crisp rifling after your initial cleaning all you'll need to is stay on top of the cleaning each time you shoot. Pyrodex can be the pits for fast developing corrosion so if its used, clean the gun the same day you shoot it. True black and T7 products are a bit more forgiving, and cleaning can be set aside a day or two after use. Just develop a good cleaning habit and you'll be fine.
If by chance the barrel is rough after that initial cleaning, don't fret too much. Pick up some JB BORE POLISH, apply it liberlly to a clean patch and wrap the patch, dry side against the bristles of a cleaning brush and give the barrel 100 strokes, changing the patch out every 30 or so strokes. Wipe clean and check then take it out and shoot it. Barrels in a world of misery can still deliver some excellent accuracy.
That Wolverine is a dandy entry level gun that typically shoots a good group to 150 -200 yards with some load development. Its a dandy deer rifle. Enjoy your new-found fun toy!
