Last year Wolfe Publishing did a special magazine on lever actions. Among the articles was one on the 444-chambered Winchester 94BB, wherein the author hunted feral donkeys in Austraila. He used the .44 caliber, 250-grain Nosler Partition. These animals were described as roughly zebra-sized, up to 600 lbs., and equally tough. Of the several taken, unspecified in the article, only three bullets were recovered. The lightest was 188.5 grains.
Now in your .45-70 a similar Partition should do about the same. For non-dangerous game in a sport hunting situation such a load should be fine. Since you're concerned with big bruin appearing at spitting distance without warning, I don't know if that would be enough penetration for all shot angles. I think 400+ grains of hardened lead would serve you better.
That 500-grain load sounds like a handful. From what you've described it may be on a very ragged edge pressure-wise with that OAL. Be careful. An extra 50 fps isn't worth a few fingers or worse.
(Edited by Bill Lester at 6:33 pm on April 10, 2001)