Just remember, if you foul up your leopard shot, chances are good that others will take the consequences. Even if you participate in the follow up of a wounded leopard, the chances are about one in four that you will be the one attacked. I wouldn't want that kind of responsibility on my conscience, just for the thrill of using a questionably adequate weapon to do a job which has to be done right the first time.

You don't need a .416 to kill a leopard, but a .257 is too far in the other direction. A .35 Whelen, 9.3X62 or its equivalent would do the job nicely with a proper expanding bullet. If you don't have one, I'll be glad to lend you one.