If you can find a good barrel it can be fitted to your extension. You will have to find a gunsmith that is experienced with Mod 12s. I have seen 20 ga model 12 barrels on Ebay. You do not need the extension and magazine parts if the one on the cgun are good, just the barrel. Good luck.

I learn something every day, and a lot of times it's that what I learned the day before was wrong