I have had single barrels done by Carlson's and am very happy with their work. They could not do a Win.101 that I had due to the wall thickness of the barrels. Go to their website and the minimum dimensions are posted.
Rose Action Sports put carlson thinwall tubes in a chrome lined barrel for me for $110.00 with 3 flush tubes of your choice. Communication was excellent. He miked the barrel first so I got the constrictions I wanted.