Perhaps a 2.4" case would get a light bullet like the 300gr. Nosler Partition moving better, within the confines of the magazine. The 2.1" case can get them moving about 2,450fps. Is more needed?

As far as I know, the one I worked on fed just fine with all bullet weights, even with the 500gr. loads. Roy didn't like to shoot those, anyway, preferring the 400gr. Barnes Spitzers with .049 copper jackets (no longer available form Barnes).

RP brass in .45/70 has a slightly rounded back edge on the brass, whereas WW and Starline brass has a sharper, more perfectly square edge. Perhaps that rounded back edge of RP brass helped the problem? Otherwise I have no solution - other than perhaps a stronger magazine spring.

We both used Speer 230gr. swaged .452" Auto pistol bullets, loaded with 15gr. of AL8 for head shooting grouse. They'd actually shoot into 3" at 100 yards and were moving along about 1,350fps - Oehler M12 chronograph 1978. I used 'pinch' of KAPOC (from a life jacket) as a filler as I didn't know any better - back then - who did? Maybe it doesn't matter in that short case.