Short chambered barrels are cut to near-final depth, requiring only finish reaming and head spacing.
To start, clamp the barreled action in a vise and clean the chamber. You're going to need a finishing reamer and a set of headspace gauges for this job.

Insert the go gauge into the chamber and note the bolt won't close. To lengthen the chamber, simply oil up the reamer, place it into the chamber, and turn it clockwise using a T-handle.

Remove the reamer frequently and brush away the chips. Now, clean the chamber again and insert the go gauge. If the chamber is long enough, the bolt will close.

A no-go gauge is used to check for excessive headspace, so the bolt shouldn't close on a no-go gauge.

Finish reaming and head spacing: You've got to have the right tools.
-- Larry Potterfield