I have an Encore barrel chambered in .500 Maximum for which brass isn't readily available. Instead of having to cut down .50 Alaskan brass, will a .50 Alaskan reamer clean up the .500 Max chamber? Any reason doing so would be a bad idea??? I have plenty of big bore Encores and revolvers so the potential extra recoil isn't an issue...

I think that will depend on your existing chamber. They share the same parent, but with the shorter case using the same bullet diameter you may wind up with a ring at 1.61"