I would suggest you consult with your PH, they really do not like magnum cartridges use on leopards at close range. if that 257 is a Roberts, it could be ideal with the right load. You want a tack driver with a low power scope that will give you a large field of view and excellent light gathering ability even an illuminated cross hair. Leopards are not tough, BUT they are tenacious, the right bullet in the correct spot is more important than the wrong high speed bullet in the wrong place. I would also get a copy of Kevin Roberts book "The perfect shot" there is a lot of very good information in it, for all species.
Bob