Stress riser is a good point.
Another stress riser would be a dovetail cut deeply, close to the bore.
Any of these risers that are in a high pressure zone of the barrel should be avoided. The pressure is the highest in the breech, as the powder ignites and builds pressure, finally getting the ball moving.
Dan, has anyone done a pressure analysis test for muzzleloader barrels, what pressures are along a given length, given load, etc?
When you look at original barrels, some fowlers and pistols have wafer-thin muzzles, and big fat breeches. The old timers knew the pressure drops as the load gets up to speed, and the barrel keeps thinning as it goes toward the muzzle end. Was this borne of experience, or science?