Hey Bert, have a look here, please

[BPTactical]

Your first guess is what I thought when I saw it. I guess it is possible it was made that way, but some research on the gun make would reveal that. I know there are muzzle loaders that are rifled, and then the last inch is back bored and they don't seem to lose accuracy.

On a smokepole that is done to facilitate easier loading. With the projectile being patched and all contact with the rifling the patch doesn't know the difference in bore diameter and has no effect on the ball.

I need to dig and see if I can find any reference to the work that was done to Tims barrel.
I love a mystery....