I have no 350 RM and have quite a few 458 Win Mag unfired cases. I'm wondering how difficult would it be to get 350 RM from the 458 with Imperial sizing die wax? Maybe in two or three passes? Lots of trimming. TIA Packy

I squeezed some .264 Win brass down to 6.5 Rem years ago. It worked fine, so this conversion can probably be done. As I recall I killed a coyote and a deer with that gun and loads before deciding to sell the gun.

If the brass is new and unfired you may be able to do it without annealing. I'd trim it to just about your desired case length first (no need to work the extra half-inch of brass). Using a tubing cutter instead of a case trimmer would be best. The case walls may be on the thick side when you get down that far, so both annealing and neck thinning may be required. Assuming you are successful in getting the shoulder pushed back to the right place then you can thin necks, trim to final length, then chamfer.

You should have no trouble making .350 Rem. brass from .458 Win. I use a file-type trim die and have made if from .264 Win., 7mm Rem., .300 Win. and Wby., and .338 Win. brass. Push a lubed case into the trim die until you start getting heavier resistance (you will have to learn by feeling as you go), add some lube, and continue. I usually get them fully formed in about 3 passes. When it is fully into the die, cut the excess brass off with a hacksaw against the hardened surface of the die. Smooth the case mouth in your regular case trimmer if necessary. You might also need to run the cases through a sizer die before loading them. I bought my trim die from CH4D at least 30 years ago. I think it only cost $5-6 back then. Other die makers probably sell them also. I tested one batch of brass I formed and got 12 loadings with only one split neck.

Forming it will be easy; it might need reaming or turning depending on your chamber.
I have made it from 7mm mag; all of my rifles need reaming to work. I have 4. I think. I like the 350.

I must have been lucky. My rifle is a Rem. 700 Classic and I have never needed to ream or turn the necks on the brass I have made. I suppose the only way you will know for your rifle is to try it.

Nosler does .350 RM brass. I secured a supply that should last me a long time. They are currently out of stock -

https://shop.nosler.com/nosler...0-rem-mag-25-ct.html

This outfit apparently has stock -

https://www.precisionreloading...rt.php#!l=NS&i=11928

Nosler wants more than I want to pay for 25 unprimed and empty brass. I already own about 200 458 WM virgin cases. The file type die sounds the best. I can thin the necks with the Forster. Thanks for the ideas guys. Packy

I made it from 300 Win Mag brass with a single pass through my sizing die. All I did was cut the brass at the shoulder and then chamfer it and use a bunch of RCBS lube. You have to trim the necks though as they will be thicker.
I had to do this when I bought my 350 Rem Mag because there was no brass for it and it took 6 months of back order to get actual 350 RM brass.