I got a response from Knight with this response: You can order a 45 cal in a 1:20 twist which is the Knight twist rate in a Mountaineer barrel which will work in the Disc stock.. These are direct drop in barrels and the barrel and receiver are 1 piece. If you do this though, you will need to change your breech plug due to the length of the one for Disc being shorter. You will be able to use your Disc bolt handle and firing pin.
At $339 plus shipping and a different BP it would be pretty cool to have a rifle that I can switch out from .45 to .50 in a snap.
But in all honesty I'd probably only shoot the .45.