I have tightened up the rear sight on a number of muzzleloader’s using this method. Works like a charm!
Another method to consider would be to stretch out the base of the rear sight its self. That means turning the sight over and peening out the the bottom side of the base edges with a punch and small hammer. Can also peen out an indentation on each corner and one indentation in the center. Between peening out the under side of the sight base,coupled with the indentation’s, that too will create a tighter fit along with friction. Also without having to alter the dovetail on your barrel.
Another wise member here told me that if you alter a part, always alter the least expensive part!
Anyway, both techniques will achieve your desired effect.
Respectfully, Cowboy