There were multiple tests and measurements that used to occur. In the case of shotguns, the barrels were tested 1st. After they passed, then the barrels fitted to the action were tested. At one time, in Liege, there were 5 separate firings that could take place. Today, I am sure that has changed.

A general discussion
http://en.wikipedia.org/wiki/Proof_mark

Birmingham proof house.
http://www.gunproof.co.uk/Proofing/proofing.html

SAAMI standards
http://www.saami.org/Publications.cfm

Some interesting reading.
http://forums.nitroexpress.com/showflat....=&fpart=all

Any instrument used to measure has to be calibrated and certified. Also it is not just the barrels that are measured but also the action and the fit or lock-up of the two.

Pete