Yup, the venerable old .45-70 in a modern lever gun is an extremely popular combination these days for use on big animals in close cover. It dates from 1873 when it was originally loaded in the Trapdoor Springfield with cast lead bullets and black powder. Original loads pushed a 500 grain bullet to about 1,200 fps and a 400 grain to about 1,350. Modern high pressure smokeless loads will push 400 grain bullets to about 2,000 fps. These are only safe in modern rifles, like Marlin 1895 and newly manufactured Winchester Model 1886 rifles. The Ruger #1 is also available in .45-70 and suited for these heavy loads as well. This is, however, a relatively short range combination, maybe out to 150 yards or so before its looping trajectory becomes a hinderance.
Here are my pair of modern .45-70 lever guns, set up just as Paul suggests, with peep sights. This works really well in thick woods and the like. I've found it very effective on the animals that I've shot with it. The top one is an 1895 Marlin, bottom is a Winchester 1886.
Here is the load I use, again compared to the .30-'06. The bullet is cast from wheel weights in an RCBS mold and weighs about 410 grains. I can get this thing going about 2,000 fps from these rifles. The recoil is absolutely brutal, though, even worse than my Ruger #1 in .458 Winchester Magnum. The rifles are somewhat light and the buttstocks are too narrow. I've backed way off on my load, down to about 1,700 fps, with no noticeable loss in effectiveness but a whole lot more pleasant to shoot.
And yes, the 12 gauge slug is an extremely popular and effective bear defense combination. No one really hunts with it, due to its range limitations, but when it's close enough that that doesn't matter, few things hit harder. Lots of fishermen carry short barreled, pistol grip shotguns on the bear streams.
There are several lengths of cartridges that share that basic .45 caliber straight case, all dating from the black powder days. The .45-70 is the shortest at a case length of 2.1". Next up is the .45-90 at 2.4", then the .45-100 at 2.6", then the .45-110 at 2 7/8", and finally the .45-120 at 3.25". All but the .45-70 are too long to cycle through modern lever guns, except the Winchester 1886 can accept the .45-90. The rest are used exclusively in single shots.
Here are the .45-70, .45-90, and .45-100. The .45-70 and 90 are loaded with 400 grain hunting bullets, the .45-100 with a 540 grain match bullet. A .223 and .45 ACP are included for comparison.
