Sure. I have a modern Win 1886 with 24" barrel. Reloading manuals group Marlin and modern 1886s together, I dont know if one action is stronger. Barnes was the only place I found that published 250g and 300g bullet data for what I wanted to do. They state they go to 42k psi in their Marlin with a 24" barrel, but SAAMI is 25k. You should be aware that different 45-70 bullets can take up significantly more or less case capacity, which greatly affects pressure. So dont assume, as I did, that one 250g bullet load will be "pretty close" for another 250g bullet. I fired loads this summer that were too hot where the lever got sticky when using a charge weight I thought well under max. (Tried my load for Hornady 300g with a Hammer 276g - Hammers take up more case volume even though its a lighter bullet...dumb dumb dumb). Further inspection showed the case heads had witness marks from the breech face and a slight swelling of circumference (about .002") just above the rim - I only discovered the last one because I made a Little Crow style brass trimmer, and all the brass with witness marks were a tight fit down by the rim. Some didnt want to go in. Anyway, back to the 250g SH, 1680 had no problem getting over 2500fps. In fact, quite a bit higher, but nothing was accurate for me. Stumped, I wanted to try a powder similar to aa1680 but filled the case up better. I eventually worked up to a load with IMR4198 (suggested by a man with way more experience loading 45-70). 55g gave 2500fps on a warm day without any pressure signs. I'm able to test at home, so I'd just walk outside and shoot across the chrono to keep the powder from warming up too much because it was 110 for 2 freakin months here this year. I went higher but backed off when I saw the first case head mark. Yesterday I got my ancient computer booted and Quickload showed about 42k psi depnding on case capacity/seating depth, temp, burn rate variance...all these variables in QL can be manuevered around to make you feel better or worse about psi, so use with care. Great tool tho. I loaded Starline brass with Fed 210s to 2.525 OAL. It's compressed, showing 110% or so. Case volume is 79g water on my cases which also matched QL.
As of yesterday I'm almost out of I4198, and there's none to be had. Havent seen H4198 in years. I ordered N120 yesterday, looks promising.
With your short barrel, I'd suggest aa1680 or N120. Start at the bottom of the Barnes data and work up...people always say that, but 45-70 has been extra tricky for me. I don't know what pressure signs you should look for in a Marlin, maybe Google it. FWIW, I had no blown primers or loose pockets, even on the stoutest loads I tried. I will try to run QL to see how much velocity you can expect to loose and get back to you.
Last thing, you may need a higher front sight unless your rifle is scoped. Skinner sights has an equation to estimate front sight height on their web page, good resource if this is an issue.
