I made & installed an approx 6" extension on a mod 97 Win pump once for co-worker. The original bbl had been cut off,don't recall exact length, but we made it to 30". I was able to put this bbl in a lathe & turn down about half the thickness for a bit & bored the extension to fit over it. I then silver soldered it on. On a double of course one could not do the work on a lathe, but not to say it could not be done by some other method. The important point will I think be how much bbl wall you have to work with.
