bumble,
We need a little more info to help out. What are you trying to do? Are you only trying to remove the rear sight ladder (on an M48 8mm Mauser)and leave the base, or are you attempting to remove the entire sight assembly? GunNut69 is correct about needing to heat up the soldering sleeeve on the barrel to remove. It is prudent that you completely remove the barrel and action from the stock and handguard before attempting this.
Jim