I have a M1899 US Krag receiver and cut barrel. I am looking to rechamber to 45-70, for low pressure, rimmed cartridge, large bore, and put a 16-20" threaded barrel to shoot suppressed subsonic, 500gr ammo. A real thumper. In theory, if you cut off the rim, created a small channel on the bolt body for the extractor to rotate on that is only if the rim could not be thinned enough to fit the 45-70, tweeked the extractor hook up to accommodate the larger rim, only .0315" higher and lower, as the rim is only .063" larger on 45-70, and kept the magazine box by either shaving down the floor plate and raising the bottom of the action .0315" to fit the rim or alternatively creating a small channel to feed the larger rim as the rim of the 30-40 is still .04" larger than the body of the 45-70. Can anyone fact check this in theory, I have no money into the action, which is in amazing shape btw, so I would be willing to spend some money on making this a unique piece.

In theory, final product would be krag action with functional magazine, thumbhole compact stock, 16-20" fluted barrel with threads for muzzle break or silencer co hybrid suppressor to shoot 100 or less yards with different loads but mostly high grain, 500gr? subsonic loads. Pressure wise, as long as not +P 45-70, 30-40 krag is still much hotter pressure wise. Additionally I already have a sporter M1896 in 30-40 with leupold 1.5-4x scout scope with original barrel. So I do not need another 30-40. Thanks for your thoughts and help.

Maybe you could reconsider the project and just mod some semi-current rifle with modern steels and call it a day?  Or perhaps purchase a purpose built 45-70?

Just thinking out loud.

I know it is much more practical to just buy a new 45-70 or make a different project rifle. But a couple things are 1, this barreled action has been in the family forever and would hate to see it go to waste, but has no book value, also cool factor has some value as well, I don’t believe there would be anything out there like what I am going for. Plus I have no money into the action other than cleaning it, it has no stock.