Gil, I would guess $150-200 if they knew what had to be done. Gunter Pfrommer would be a likely prospect for that job if you explained what had to be done. I did the process on a circa 1913 20 gauge nickel steel gun with 2 1/2" chamber and it worked well. Strange thing about the M-12 is that even the 3" heavy duck model has an ejection port that is not even 2 3/4" long ,as I recall. You can only open it to the edge of the chamber ring maximum, but it works. Regards, Marcus