I imagine almost any expanding rifle bullet in the 150-180 grain range out of a .308 would kill a leopard very effectively if the shot is on target. After all, a good sized male leopard only weighs about 150 pounds. I mentioned Craig Boddington's comments on leopard bullets with reference to his suggestion that bullets that expand fairly easily may give a quicker kill than a tougher bullet. He specifically talks about the importance of this with .375 bullets and suggests that .375 bullets that are ideal for buffalo and other large game may not be ideal for leopard and that a lighter, softer bullet is better.
In cartridges that are commonly used for deer sized game, like the .308, the difference between tougher and softer bullets is probably not so important. But if you're looking for the ideal leopard bullet in a given cartridge, including the .308, it could be worth considering.
Obviously, all this nit picking about bullets is very secondary to shot placement, which is the key to a quick kill, on leopards or anything else.
