I have had a few Mountain Guns on the bench, and I knew they weren't cut on 11° but I didn't know what the angle was. I found out today from S&W that the 44 Magnum is cut on 8° and the 45 Colt is cut on 10° and that set me to thinking.
We know that using a Taylor Throat Reamer reduces pressure because it lessens the resistance against the boolit entering the bore. I am speculating that S&W used 8° on the 44 because the barrel stub is thicker due to the smaller bore, and they used 10° on the 45 Colt for two reasons. One, the threads on the barrel are the same OD between the two calibers but the barrel stub is thinner on the 45 because the 45 bore is larger, and using 8° would weaken the barrel, and second, they also know that the 45 Colt is typically loaded to lower pressures than the 44 magnum, even at the "Ruger Only" level it is generally less than the 44 magnum. This is S&W's way of reducing the pressure at the barrel/frame juncture.
Irregardless, I am not aware of a manufacturer that would make the D-shell style forcing cone cutter that is sold in the Brownell's chamfer kit in anything less than 11° Might get away with using the 11° angle on the 45 Colt Mountain Gun, but that won't work for the 44.