Superb... I love it. I hope it is the only. 20 out there!!
I would etch the details of the calibre change somewhere on the action like Mr potterfield dies at midway... that way in 100 years the story won't be lost!

Here's a picture of mine. I got given this by my grampa 25 years ago and I treasure it... shot thousands of pellets through it! Just put a prosystem silencer on it thanks to me hsing-eee from Dundee, and am in the process of sorting out the power from it's new gas ram.