Checking concentricity of a barreled action

Copied and pasted from link below: Concentricity of a barreled action It is well known by most of us that gunsmiths have a knack of knowing when a barrel is straight. That thoughtful examination of the bore against a backlight keeps us in suspense and has always intrigued me. How do they do it, how can they do that.

This time my gunsmith was kind and patient enough to teach me, and there in front of me those concentric rings appeared, a revelation. A pin-hole plug on one end of the barrel creates reflective concentric circles inside the bore, simplicity at its best. A warped barrel was easy to tell. However for even further sensitivity in his 'read', my gunsmith rotates the barrel on a special barrel spinner while studying those concentric rings.

Back at home a search on the subject brought up a 1975 patent that goes into the details of how and why this technique works. It is provided as reference at the bottom of this page.

A poor man's way is to chamber a deprimed case and look down the muzzle end, allowing examination of a barreled action. Coaxially rotating a barreled action is very difficult, so a better way had to be found and Photoshop came to the rescue. The spinning part of the process was done digitally, on my computer screen which I describe next http://65br.com/page1.htm ....