That side by side is a shotgun. When you miss, it doesn't really matter what you're shooting, and a shotgun pattern at that distance is no larger than a rifle. I've killed two leopards, and fortunately both were clean kills with no need for that type of follow up. If I were to follow one up, I'd be carrying the Beretta shotgun I've shot thousands of rounds through, but only because I shoot it like an extension of my arm. On a Botswana lion hunt in 1989, my 12 ga. Beretta was carried by one of the trackers every day that we were out lion hunting. After doing it, the only thing that shotgun would have been effective for would have been inside of 10 yards, or for shooting us to put us out of our misery before the lion did.
All the PHs that I know carry heavy rifles when following up leopards. One friend, Alan Vincent, killed two wounded leopards in 2006 or 2007, and he finished both with his .458. They carry those rifles every day, so they are the guns they know and shoot best.
In a charge, I'd be more worried about getting shot than killed by the cat.