I would be interested in the comments and thoughts of the professional writers and/or reloading professionals on the following.
I was loading up loads in 45-70 and 45-60 for a Marlin ’95 and an Uberti ’76. These were intended to be “fun” loads for knocking over steel plates at 50 yds.
The loads are generally in the 1200-1300 fps and use of SR4759, 4198, 5744, 2015, and H4785. I am using 300 gr. cast bullets with gas checks for both rifle loads.
In looking for some reduced loads in the same fps range for the 45-70 in the Class I (Trapdoor pressures) I noticed a considerable difference in powder amounts for the same powder between the 45-60 and 45-70, which aside from the length of case are the same caliber and bullet weight.
The load books caution the 45-60 loads are intended for the new manufactured ’76 clones and not for 100+ year old Winchesters. Still, in looking at the pressures that are given for the 45-60 and the Trapdoor Class I 45-70 (with the same bullet) the generated pressures appear to be in the same range of PSI.
So aside from the length of case and empty space, what is the mystery to account for the difference of (for example) SR4759 beginning load for the 45-60 (in new rifles) and the beginning SR4759 load for the trapdoor 45-70?
In my 4+ decades of reloading, I’ve followed the load book guides and accepted the need to account for pressures and pressure spikes, but this has me scratching my head to account for the difference.
I’d appreciate the learned writer’s insights and thoughts.
