H4895 can be reduced to 60 percent of maximum per Hodgdon data, anywhere that powder is listed. IMR 4895 has those same attributes, as it closely replicates their original (the Hodgdon version), as do many of the "older" single based extruded powders.
H4198 and I4198 are two other powders I have reduced with success, but not in that case volume (I use them in the 416 Rigby and 45/70).
Actually, DuPont made the first IMR-4895. The Hodgden version was just milsurp that they bought up and mixed to give one burning rate.
I've been trying to figure out just when they first made 4895, and my best gues is around 1937 as a powder to load ammo for the M-1 Garand. At one time, reloaders could buy powder through the DCM (Now called CMP) and when 4895 was purchased, it would come with a data sheet that said, "Use data for 4198" or 4320, or 4064, depending on the lot number. at one time, data for IMR and H-4895 was for all practical purposes interchngeable, although not so today.
(Research for the above was from Phil Sharpe's Complete Guide to Handloading, Earl Naramore's Principles and practices of Loading ammunition and all copies of the American Rifleman from 1936 through 1950. I have copies up to and including the latest one.)
As near as I can figure out, DuPont upgraded the IMR powders in the 1936/37 general time frame.
Paul B.
