I am looking for safe loads for this rifle. I just recently found some IMR 4759 powder. I don't really want to use anything over a 300gr. cast bullet. Does anyone have the knowledge of loads for this rifle?

That powder has been discontinued quite a while, but all the older Lyman manuals listed it for cast bullets in almost every rifle cartridge. You can probably Google up some Lyman manual pages, given how popular 45-70 has been the past 20 years or so.

I have an old Lyman Ideal Handbook #40 from April 1965. It lists the following load for 45-70 with 4759 and 300 gr cast plain base bullet:

25.0 gr yielding 1565 fps

Use at your own risk, of course.

Here's another data point from Speer #13, albeit it's for 350 gr jacketed.

If you have any pistol powder on hand, I'd look at this data too:

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

They have low velocity .45-70 data with a 300 gr cast boolit

If the OP intends to use cast bullets, he should be certain to slug the bore and obtain soft bullets at least 0.001” greater in diameter. Trapdoor bores have a wide range of diameters, and the common hard 0.457” bullets may lead badly in bores measuring up to 0.463”.


.

I would start at 28grs and work up to 30grs


My 45/70 likes 30grs with 300gr cast and 28grs with 405 cast.



Here is an excellent source of low velocity data: http://www.gmdr.com/lever/lowveldata.htm