Checking for a bulged barrel is not as easy with a muzzleloader as it is with a modern rifle.
The chance that the barrel is bulged is much higher with the muzzleloader because all it takes is to leave the ball/bullet partially rammed down the bore and then to fire the gun.
Modern rifles thin barrels will easily show a bulge but the heavy barrels on muzzleloaders can bulge to an unacceptable level and look quite all right on the outside.
Before one goes out shopping for a used muzzleloader they need to have a cleaning rod, a brass cleaning jag in the caliber(s) they are interested in and a few clean, very lightly oiled cleaning patches with them
Tell the owner you want to check the rifle by running a clean patch down the bore. If he objects then tell him how to insert his gun in the proper place.
When you run the correct sized cleaning jag and oiled patch down the bore pay particular attention to the amount of force that is needed to keep the jag/patch moving down the bore.
If at any point it becomes noticeably easier to push or it gets easier then returns to the previous force the barrel has been bulged.
Not only is this possibly dangerous to shoot but even if it is safe it will never give good groups.
After your patch is back out of the bore look for rust or black fouling.
Light rust isn't necessarily bad but heavy rust or black fouling indicates the bore wasn't cleaned and is probably junk.
Happy Hunting.
