Most of the big drops in fps out of long bbl guns being cut back is due to the powder charges and burn rates of the slower rifle powders. You may experience a increase depending on the powders burn rate. If it is fast, and the powders pressure is spiked by the time it gets 12 inches down the bbl, that would mean you would not lose any bullet speed due to bbl friction.
There is no real answer, I would however look up estimates from a TC Contender with a 14 or 16 bbl, just to give you a good ball park figure. Though a 16 is legal add a 1/4 inch incase you ever have to have it recrowned. Do post some pics when it is done and a few fps figures before and after. Thanks ahead of time, good luck on your project.