Break-a-way, sold at auto parts stores will do just that: break a tight joint away. If somebody has applied red loctite to a sight, you may have to put the copper tip of your Weller soldering gun (any hardware store sells these) and apply heat for a minute or two with the trigger full back.
One of the more expensive (around $500) sight fixtures will also do the job, if you are a rich Pistolsmith like Dane.