MSP Ret, In a bottle neck case, throats are of greater degrees in an effort to control throat erosion. The 45-70, while burning powder charge weights consistent with the likes of a 25-06, it is not burning them at the same pressures, hence we don't see the same flame temperatures, and don't see the erosion effects as quickly as a 1.5 degree throat show erosion in a case like the 25-06 or 243 for that matter. I've done a pile of questioning on this over the last several months, and with chamberings like the 45-70, 44 Mag, 444 Marlin and similar, it seems there is more to be gained than lost. At worst, ones rifle simply won't respond to the treatment.
Early on in the advent of Marlin using Microgroove rifling, it was quickly found that those barrels did not shoot cast bullets at speeds above 1600 fps or so at all. The cure was the 1.5 degree throat, so that the bullet was not being subjected to huge swaging changes in its transition from the case to the bore and a better alignment with the rifling. Of late, I have been reading of bullets being sized to fit the throat of a chamber as a means of not only preventing leading, but of increasing accuracy as well.