Chuck,
I have a Fox 16 that I chamber sleeved down to a 20 bore. The chamber were ruined and I wanted to save the barrels without major expenses. As I had several 16 with 30" barrels I decided toi try to make a set of 20/16 barrels. It is choked full and full and gives full and full choked patterns with 20 shells. My chamber sleeves are longer than the chamber area as I turned them on my lathe to enter the bores by 1 3/4". They have a slow taper outward to the 16 bore. I know that I could have made them longer to perhaps get a better seal and burn out of the 20 shells but the short chamber sleeved replacements work well. Crony reading are almost exactly the same as my 20 Fox with 28" barrels. Maybe 30-50 fps difference.
I had to drill out both chambers because they were so pitted that it was not safe anymore. Storing two paper hulls for 50+ years, with high humidity, caused the shells to crumble and they in turn caused the chambers to pit. I ended up taking almost .035 of the chamber wall thickness out in one and .030 in the second. Then made a chamber casting of both chambers and first two inches of the bores to figure out how to make the chamber inserts. Wasted a ton of metal on the lathe and milling machine before I got two that fit as I wanted them. It taught me a lot about machine operation and just how difficult what seemed like a simple plan could end up as another 100+ hour education.
So in effect I have a over bored 20. I did pattern it and found that it patterned one ounce loads slightly better than it did the factory AA's 7/8 I used for a base line. If I had to do it all over again I would just go back as a 16 or even find another barrel. I have the correct chamber reamer available and figure that my custom barrel chamber making days are not a real good use of my time. Then again if I came across a 20 with bad chambers I might try to make a set of 28 chamber sleeves with about a 10-12" barrel insert. Be a fun gun to shoot skeet with and maybe a few released quail.
