Let me tell you a story...
One of my clay pigeon shooting pals is now in his 80s. During the 1970s he had built for him a Best quality 12 bore sidelock by A.A. Brown. Those who know will know the quality of his work.
Circumstances changed and he needed to sell it. His gunsmith advised that the best he could expect for it was £2,000 (quite a lot then). However he also said that if he agreed he could open out the chambers from 2.5" to 2.75" and have it reproofed, which might make it more saleable. BUT if it failed Proof then it would be scrap.
So he agreed to have the chambers lengthened, and the gun then passed proof - phew.
It then sold to someone in the USA for £5,000, or about $12,000 at that time.
He now shoots one of the last Webley 700s plus a Miroku MK60 20 bore.
HB