Without question the most famous lever action wildcat is the .450 Alaskan, developed by Harold Johnson at Coopers Landing, Alaska.
To get solidly on the right side of the margin for the largest game with a lever-action, the solution is wildcatting. There are wildcat cartridges up and down the spectrum, but in the lever-action world I don't think they make much sense until you get up to the top of the food chain. Then, well, it depends on how much you love lever actions, doesn't it? Without question the most famous lever action wildcat is the .450 Alaskan, developed by Harold Johnson at Coopers Landing, Alaska. It's the .348 Winchester necked up to take a .458-inch bullet, and there are loads out there that suggest you can get a 400-grain bullet up to 2200 fps. Designed for backup work on Alaskan brown bear, it's truly powerful enough for anything (given the right bullets).
Professional hunter Willem van Dyk and Boddington with a great Cape buffalo from coastal Mozambique, taken with the Turnbull 1886 in .475 Turnbull. Yes, it was plenty of gun.
I once owned, briefly, a Winchester Model 71 rechambered to the blown-out Ackley version. That one propelled a 400-grain bullet at 2150 fps, which neatly crosses the 4000-ft-lb barrier, and thus was street-legal for the largest game in the world anywhere in the world. However, with its half-magazine and rebored barrel it was much too light, and I think it was the hardest-kicking rifle I have ever owned. I was also nervous about the pressure; it was impossible to keep the lever closed when it went off.
Both the Model 71 and its parent action, the '86 Winchester — especially the newly manufactured versions — have again offered a fertile ground for wildcatters. Lots of cartridges have been experimented with in virtually all of the "over .40" bullet diameters. I have no idea which one might be best, but the one I know best is the .475 Turnbull, developed by Doug Turnbull of Turnbull Restoration and Dave Scovill of Wolfe Publishing. They took the old .50-110 case and necked it down to take a .475-inch bullet, then got Randy Brooks at Barnes Bullets to make runs of flat-point 400-grain TSX and Super Solid bullets. Ideally, this bullet is a bit light for caliber, but we have long figured out that homogenous alloy bullets like these make it possible to drop weight (and add velocity) without sacrificing performance.
For the really largest game it is almost mandatory to go the wildcat route. The .475 Turnbull, based on the old .50-110 case, takes energies well over the 4000 ft-lb minimum required in some African areas, and has been used on both buffalo and elephant.
The loads I'm shooting propel the 400-grain bullet, solid or soft, at 2150 fps, yielding 4104 ft-lbs of energy. Lighter bullets of 350 grains will be faster, and heavier bullets (450 or 500 grains) will be slower, but across the board the energy figures run from 4000 to 4200 ft-lbs, keeping the .475 Turnbull legal for dangerous game anywhere in the world. Honestly, when I got the rifle I was scared to shoot it, remembering the beating that old .450 Ackley Improved Alaskan gave me. This is a different kettle of fish. The barrel is longer and heavier, with a full magazine, and Turnbull wisely added a non-traditional recoil pad. It's actually a whole lot of fun (and not much pain) to shoot. It is also incredibly accurate, though limited a bit by the traditional buckhorn sight.
I used it to take a Cape buffalo in Mozambique last year, and it performed marvelously. Hell, I liked it well enough that, rather than return the rifle, I sent Doug Turnbull a check — which I never intended to do. Colleague Scovill (and others) have used it on game up to elephant, and I have no problem with the concept. So, to my thinking, the ultimate big bore lever action for the ultimate big game definitely exists — but it isn't available in factory form. Perhaps it never will be.