Geno,
A hot 45/70 double rifle would be quite effective on bear, since you'd be getting loads comparable to the 450-476 nitro cartridges. My only question would be regulation. I have a SxS in 45/70 which was regulated for the standard 405 grain loads and groups them very well. It's death on driven boar. However, hotter loads fly all over the place. At very close range (i.e. in dark den), it probably wouldn't matter, but it seems to me you'd need to have a rifle built and regulated for the hot 45/70 loadings if you wanted to use it more generally. In which case, why not just get it chambered for one of the nitro cartrigdes? Low pressure, slightly increased ammunition availability and aplicable from pig up through elephant.
