Never hunted Leopard but I would choose a rifle every time. I think the reason some people choose to use buck is that they think it will give more lee way when aiming due to the spread when in fact the pellets will still be very close together at the ranges normally needed in a Leopard charge. A single projectile from a rifle would penetrate much better then a group of shot and is also going much faster which is supposed to impart a fair amount of shock on the very shock suceptible cat.

In fact I would be a bit nervous if my PH took a shotgun to follow up the cat.