Rifle or revolver/pistol barrel?
To fix what problem?
In my revolvers which had a constriction at the barrel-frame union I've hand lapped out the constrictions (~.0010"-.0015") using a very tight cotton patch (usually several layers) on a cleaning rod, with a generous amount of abrasive compound on it, & concentrating action in the area of the constriction.
Extra elbow grease is optional.
The constriction is detected using a set of pin gages by confirming smooth passage of the largest gage thru the whole length of the barrel.
Repair is confirmed once the gage passes the constricted point.
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