backbore

To modify (a weapon's barrel) so as to increase the bore diameter.

To bore from the backside of a workpiece by using a boring tool that passes through one or more through holes, deploys a cutting edge (or receives it by manual insertion from an operator), bores a feature on the far side, retracts the cutting edge (or has it removed), and then retreats back through the hole (toward the tool's origin).

The section between the throat and shank of the mouthpiece of a brass instrument.