I cannot claim much recent experience with lions. And given the current cost of a lion safari, who can? I was fortunate to do my lion hunting back in the '70s and '80s, when it was less specialized, more available and much less expensive.
A leopard that is shot over bait will almost always move away when hit, giving you time to regroup. Lions are different; they can go from sleepy to ferocious in an instant. If you do your job there will be no charge, but you must be properly armed and ready.
A lot of new rifles and cartridges have come along since then. Even so, my opinion remains unchanged: The ideal lion rifle is a scoped .375. It is plenty of gun to start with and enough gun to end with. Note that I didn't say which .375 because I don't think it makes much difference. The .375 H&H is the classic lion rifle, but the .375 Remington Ultra Mag, the new .375 Ruger, the .375 Weatherby and all the rest are perfectly suitable.
One important note: Lions are thin-skinned animals, and you want a bullet that will get in there, open up and really wreck the vitals. The toughest bullets are not called for and may well be dangerous. I shot my best lion with a .375 H&H, but I was using a tough, extra-heavy 350-grain bullet. It didn't open up, and we had to follow up farther than should have been necessary.
Another time we chanced across a fine lion while buffalo hunting. I was carrying a .416 Rigby--certainly a good choice but the softpoint up the spout misfired. I worked the bolt and shot the now-alerted lion with the first cartridge in the magazine, which was a 400-grain solid. The shot was perfectly placed, but we still had a wild melee to sort out. It is unlikely that I will ever hunt lion again. If I do I may or may not stick with the .375, but I will absolutely use a bullet that is certain to open with minimal resistance.
A lion is not bulletproof, but even though the size is consistently exaggerated, a lion is a lot of cat. He is also extremely courageous in the charge, and if he has enough strength left to kill you he will, no matter how severe his injuries. A .375 is enough gun to stop a charge, and a scoped rifle is a far better choice for the opening salvo. Only an idiot would take a longish shot at a lion, but a 100-shot is not excessive. It may be in poor light, dawn or dusk, so a scoped .375 is a fine choice.
In the case of a charge, no rifle is too big, but if the shot is well placed, an expanding bullet from a .375 will stop him. The trick is to hit him and hit him right, and this is very difficult. I have stood two lion charges--not a lot but two more than I ever needed. The lion is a much bigger target than a leopard, but he comes low and fast, often tacking as he comes, until the last moment. Cool hands hold their fire until the end, but you can't wait too long or he will still hit you. Optimally, I suppose, a bigbore double would be the best choice. But it is probably not the best choice to start with, and there may not be the option to switch.
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