There are several reasons for re-boring a chamber. A tight chamber is one reason. Another reason could be a slight change in caliber ( like from .45ACP to Cane's .460 Rowland ). Another reason could be that you have a reliability issue for whatever reason ( stuck cases after ignition for example ), and opening it up the very, very small amount .001" in diameter, could help with that.
We don't know why Castle's dad did this. He had a Colt, that could be reason enough ( ZING!! The Ref is going to have to step in here and warn Dillinger as Cane is clearly hurting from that illegal blow
I presume that some script writer needed to talk guns, went to some other stupid forum and got bad gun advice.
If these guys would just bookmark us, they wouldn't have this problem with their work.