The .45/70 was origionaly a Black Powder number. In this case, at least, the taper is not what amkes it low pressure. The .45/70 can, and HAS, been loaded to much higher pressures, but for the Conyender all the literature I can find says to use the old BP equiv. pressures (i.e. Trapdoor rifle pressures.) Hope this helps a bit.
Brian M.