For me, I won't go over 50gr of IMR3031 with a modern Marlin Lever with a 405gr lasercast. And I won't go over 56gr of IMR3031 with a 300gr. So for you, I wouldn't go over 45gr with your 425gr. Beleive me, it will start jarring your teeth when you get to 45gr.

Since 425gr is such an oddball, I would call the vender you bought it from.

I have Oregon Trail data for a 430gr gas checked cast and they say in modern guns 45gr-52.5gr with velocity at 1900fps at 52.5. But holy cow, after 45gr, thats a whole lot of beating your shoulder.

I suggest starting at 40gr, and work up to 45, and see how you like it. If you really want some more bang bang, next time out, try some more.

IMR3031 is such a slow burning powder for the 45-70, its fairly safe because you usually run out of case space before you go dangerously overpressure. Out of Handloader Mag Aug 2003 edition,Brian Pearceclaim is modern Marlins can easily handle 43.5KPSI and 444 Marlin can handle 44,000CUP. Where a Ruger #1 can handle 50KPSI or more. So there is a big contraversy, about 45-70 data.

According to the article in which I have the upmost confidence in Brian's info, Marlin Model 1881 models can only handle 20,000CUP or the same as 1873 trapdoors. And that cooresponds to your typical factory loads. A pre nickel winchester 1886 loads should be limited to 28KPSI. Marlin 1895 (made 1895-1915) should also be limited to loads 28KPSI. Smokeless era Winchesters 1886 with nickel steel barrels can withstand loads 35K-40KPSI. Post war levers such as Marlin 1895 (1972 to present) and Jap brownings can withstand loads 43,500PSI. So you can see why the various mfg. take cautious approaches to data.

Hornady, Nosler, Barnesand even Sierra has fairly hot loads for the the Modern lever . Lymans, and speer, take the very conservative road when it comes to lever and does not differienate between Pre 1972Marlins and post, and thier loads are very low velocity. But according to alot of gun writers like Brian Pierce, there is a difference, where Pre 72 Marlins can only operate up to 28KPSI.

In that artical he lists 430gr Oregon Trail with H-322 50gr pushing 1943fps and does not exceed 43,5KPSI. Thats some serious power. He also lists 53gr of H335 with a 430gr Oregon Trail cast at 1841fps and does not exceed 35KPSI.