Hi IvW,
I'm trying to think through this logically - and candidly am having a bit of a challenge there. The .500 is a great stopper. Well known, highly respected, it's more of the 100 yards or less weapon with Irons.
I keep thinking it's sister barrels should be something a bit more long range friendly. A .375 H&H topped with a 1-6x scope would make for an amazing pair and still be DG Legal. Further, you wouldn't have to search particularly hard to find ammo for it.
The part that makes it hard is that I really love the 450/400...but I just don't quite see how it fits if you already have a 500.
BTW, I am super happy for you and this choice! I'm sure you're going to have an absolute ball with this no matter how you choose!
I purely want this double for back-up situations. I have used my 500 JEFF for many years and it is the most devastatingly effect caliber I have ever used or seen used. I have had to use it quite a few times. Touch wood so far, every time I have only pulled the trigger once and that was all that was needed. Some of these where very close calls, 3 buff(1 cow 2 bulls) at 2 @ 7 yards 1 @ 10 yards, Male lion at 10 yards, 2 elephant at 10-20, others further away with the same results.
I will still use my 500 Jeff as my primary DG back-up rifle, however when circumstance dictates, I can then switch to the double.(Hunting tusk less in herd situations, having to follow up into thick bush after a buff, elephant etc. has been wounded.). For more open terrain nothing beats the 500 Jeff for back-up. Thick stuff close up is a different story. You basically have one shot with a bolt action if it gets very close.
The reason I want the double is that on some of these charges, had my first shot not been true, I may not have been typing this. When things get real tight in thick bush, there is often not enough time to reload. With a double you have that instant second shot available, If you can't stop it with two I doubt you would have the time with a bolt to get three in anyway.
A 375 H&H Flanged Magnum with a QD scope is a great idea for a hunter but a bad idea for use as a back-up rifle. It is just too marginal for that. A scope on any back up rifle is a bad idea. I may consider a Aimpoint in the very small model but not a scope. For leopard this caliber would be more than adequate but for a large male Lion, I am not so confident with that. I think one of the two .40 calibers would be better.
As for cats, you don't need the power of the 500 however the 450/400 is much lower in recoil and muzzle flip which would greatly increase your recovery time for the second shot. There is a big performance difference between a 500 and the 450/400 NE.
I once had to stop a large Male lion that had been wounded by a Spanish client using a straight pull Blazer rifle. Long story short he had shot it too far back(we had been tracking two males for quite a few hrs), at a distance of 35 yards of the sticks, the lion gave us a spectacular show as he made somersaults all the while growling and biting at this thing that "stung" him on the side. I told the client to reload and shoot as soon as he had a shot. The lion stop his antics and as soon as he saw us his tail went straight and with a deep growl, he charged. No I have faced a few charges before but this huge lion came at us at an astonishing speed. I kept shouting shoot all the while keeping the lion in the sights of my 500 Jeff. Out of the corner of my eye I could see the client was doing something with his rifle(that was still on the sticks), but not shooting. The clients rifle disappeared out of the corner of my eye and the lion now having reached 10 yards gave me no other option but to shoot. The shot was true and crumpled the big cat. As for the client, well he had dropped the rifle on the ground and disappeared! Further to the right of his rifle lay two unfired cartridges, he kept reloading and not shooting. The tracker found him 30 minutes later in the top of a "Geel haak" tree. The truck was a long way away and it took us a long time to get him out the tree!
Now I am jabbering but if this happens again, I would prefer a nice low recoiling double(not fitted with a scope).
Leopard is a tricky customer. Where a lion will normally try and put distance between himself and the hunters after being shot and wounded before making a last stand, a leopard will not go far. A lion will normally come from further our and 9 out of 10 times growl or at least give you warning when he starts his charge. A leopard will wait till the last minute, until you are almost on top of him before he launches an attack. No time for scopes and re loading. The only chance you have is a double rifle for the second shot. I have use a s x s shotgun with Brenneke slugs and have a 9.3 x 74R/12ga combo.
I think a double 450/400 NE would be better. Both for lion and leopard.
Would also be very handy to have the option of loading a solid and a soft at the same time when doing guided photographic safaris. If you then bump into some lions at close quarters you could silently open the double and replace the solid with another soft and vice versa when you get close to elephant.
I really like 375 H&H and have two bolt action rifles in this caliber. I also like the 9.3 x 74R but both might be a little bit marginal for my needs.
This is part of the problem, having to decide. I need to do that before I order the rifle as the second set needs to be fitted at the same time and cannot be bought afterwards.
Just checked now, that was too long for a post, apologies for that!