What type of sight, and what's it mounted on/in? For the standard 3/8" dovetail arrangements - Marlin 336 or Winchester 94 for example, I clamp the barreled action in a padded vise, with the sight's dovetail pretty much centered in the jaws and level with the top of the jaw, then use a 3/8" hard maple punch going from left to right to break the dovetail cut's grasp on the sight. From that point it's just a matter of fairly gentle tap-tap-tap till the sight slides out. I've found the hard maple to be tough enough for this sort of thing, but being softer than metal it's more forgiving. Best part, I've lots of maple scrap, and can fashion punch shapes as required for just about any task.