While what Ed said is correct this is not the limit of what has been done. Bbls having a mild choke have been "Jugged" in order to give them an increase of choke. The process is essentially as Ed described, just it is not an absolutele requirement to start with a Cyl bore.
Proofing (British) measured the bore dia at a point 9 inches from the breech, jug chhoking was all ahead of this thus did not affect the proof of the gun.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra