I have a couple of Argentine Mausers sitting here and was wondering how they would work for a .458 Winchester build. Both need a bit of welding on the ring so heat treating will be a must before I begin the build. One has an undisturbed crest and the other has been drilled and tapped for mounts. Can I use the existing bottom metal or should I go for aftermarket bottom metal? Different follower required? How much cutting to the action frame?

David Miller built many a .458 on Argentine Mauser actions. Most of the photos you see of Miller's guns show that he used Blackburn Bottom metal on his.

I don't know, so this is just a guess, but I would think that a bottom metal unit designed for a 7.65 Argentine Round would be a pretty tight fit for a .458 Win Mag. round without a lot of jury rigging.

Looks like it's about 1/16 or so too short. It could be opened up I guess, but like you said, it's a bit of work to do.
Thanks

Opening a Mauser box forward is a simple procedure. Takes a minute with a hacksaw to saw down the box at front wall. Little time with pliers and a torch to bend wall to front and weld up edges and top of front wall. A little file work and done. Last one I did was on a lever floorplate original Mauser to go on a Westley Richards single square bridge that had been converted to 375H&H. Makes a neat job.

Zimbabwe

What about the "stackability" part of it. You are going from a "normal" round to a much thicker round. Does this cause any sort of feeding problem.

It is the taper that matters most. you can have a narrow stack as long as the taper rate of the box is right. Some action cartridge combos can't ever be made to the ideal mag box due too width restrictions. where you end up with feed issues due to improper stacking is usually when you go to a more straight walled cartridge than what to box was made for. keep things about the same taper rate and the only real issue is magazine capacity.
Don

I had 416 Taylor built on an '09 Arg action that feed with no problems with the original bottom metal. It did appear to have been modified and it held two in the box.

Was a great cast bullet rifle.