I was a consultant for GaugeMate for a number of years and was responsible for sending the adapters and ammunition to Tom Armbrust/Sherman Bell for testing. The primary reason for seeking their assistance was a question that had been raised by one of our clients.
I had done velocity (Oehler 35P) and patterning work with our 12 to 20-g adapters and was very confused by the velocity readings. The readings for 20-g Win AA (with a nominal velocity of 1200 fps) were usually around 1300 fps. As basically a 'rifle guy', I assumed that I was doing something wrong. The patterns were very uniform and dropping dove or powdering clay targets was no problem. In my 'rifle world' this was like using .25-06 ammo in a .30-06 and getting increased velocity and one-inch groups. It simply didn't make sense.
As a reader of the Double Gun Journal, I was familiar with Bell's work and decided that he would be a much more credible source for our client(s). His test results have been posted on the GaugeMate site for three years. Basically the results were that 20-g ejecta comes out of a 12-g barrel at a higher velocity than it does from a 20-g barrel. GaugeMate clients routinely reported similar results. Some of that feedback is also on the GaugeMate website.
About the same time, I visited the Federal Cartridge factory in Anoka, MN. I had the opportunity to discuss the 'velocity' question with Gary Svendsen who was then their Director of Quality and Product Service. Gary was not familiar with chamber adapters and said that Federal had not tested them. I explained the increased velocity readings we had experienced with 20-g ammo in a 12-g gun. He didn't appear surprised and suggested that the likely reason was reduced wad drag.
I hope this information is helpful. I should also note that I have not been associated with GaugeMate since 2007.
Steve Helsley
