PSI and CUP are not the same units at all, but 28000cup in the 45-70 seems to be close to 28000 psi. Not the same with other ammo.
You can find helpful datas on the 45-70 in the special issue of Rifle magazine call Legacy of the Lever Gun. Go to Rifle web site, may be they have some left to sell. Interesting reading.
The russian action is strong, used for ammo which are close to 58000psi and more without problem.
Marlin is given to 42000psi, 1886 with nickel steel barrel or reproduction can go to the same, even a bit more but as the saying goes: "better safe than sorry". No need to try to transform a 45-70 in 458WinMag.
I load for my Winchesters and Browning 86 with VN133 powders and bullets from 300 to 405grs.
57,8grs of VN133 and Hornady 350RN bullet give me 620m/s a bit more than 2030fps, barrel is 22inches. Pressure is around 30000psi may be 31000. Cases are Remington, primers CCI.
Be careful not to mix cases, Remington and Winchester are close but the Federal are very heavy and capacity way smaller so same loads give over pressure.
With same powder and 405grs bullet i use 53grs for 580m/s. Pressure are close to the former above. Think you can use them without fear in your double.
Lead bullets are a must in this caliber even with smokeless powders, if you push them a bit use gas checked one and lubricate with Alox or another high quality grease or lubricant. I get very few leading doing that.
I even used 540grs GC lead bullets in my lever action. Could be interesting to test in your double. Maybe you can load it longer than in a lever for more powder capacity. Good for close range hunting on big boars if you can get them to shoot.
Have good time loading and shooting, take care!
Dom
