Was wondering if anyone has had any experience with SR 4756 powder (old DuPont stuff) in 45/70. Have a couple of pounds that need to be used and was wondering if I was waisting my time. Have heard that the new stuff has a different burn rate than the old stuff.

Also, have used 30 gr of Alliant 2400 w/300 gr Hornady FNHP with great results for a reduced load (45/70 1895gs), but noticed that the Sierra manual lists loads with 300 gr FNHP and 2400 all the way up to 2100 fps with around 40 gr. Was wondering if anyone has had any experience with 2400 and upper end loads? Accurate? Recoil? Was trying to save some time experimenting, and I like the idea of 1 powder for all levels if it works out (currently using 58gr IMR 3031 and Hornady 300 gr FNHP for upper end loads).

I have the old Sierra Manual "second edition" 1978 and still use it , but only to compare loads...Things have changed since then and so have powders... Still have cans of Hercules Bullseye, Red Dot, Unique, and 2400 in my stock.I only use it for "special loads"...

I use H4198 with a 405gr. hard-cast now for loading, and it works for trapdoor to full- on sholder thumpers...

http://www.gmdr.com/lever/lowveldata.htm

SR4756 has a burn rate similar to VV 3N37 and Solo 1250.

I am not familiar with the 2400 loads that you described, but if they are from a reliable source you could consider using them.
I have used 2400 quite extensively with 405 grain cast bullets at 1500-1600 fps. I find that this is the optimum pressure range with 2400, with good, consistent, velocities and good accuracy. Loading manuals do not indicate higher velocity with the 405 grain cast bullet, so I don't try.
Cannot speak for 300 grain jacketed.

I've used 2400 for mid-range (~1400fps) loads, for fun and plinking. I'm not sure I'd go to hot loads with it; there are better powders out there for hot loads that won't risk high pressure the way low-volume powders can. I use Unique with cast bullets at 1100fps. Best accuracy has been the slow loads with Unique, and H335 and IMR4895 hot loads; 2100fps with 300gr Sierra HP's to 1700fps with 405gr Remingtons, and various points in between. It can go hotter, but my shoulder may not.

The nice thing about the 45-70 is that it's versatile; you can load it hot or not. Just follow your manuals and watch what powders generate high pressures and at what load levels.