I did a search and all the old posts from the last two decades point to dead ends.

I need the full length version, nothing shortened, just a 375 Ultra Mag necked up to 458.

Tony Rumore
Tromix

Too easy; chamber with a piloted 375 R reamer. Neck and throat with most any 458 reamer.

Back around 2010 I bought one from Midway.

Be sure to get one with the changeable pilot.

One I got was a .300RUM, it's tricky doings
but I turned a .358" collar for mine.

Much like turning the center band on an ink pen.

Had the barrel by the lathe to check the fit with. Once it was done, parted it off the shaft.

Once you have one, you can make or have someone make the collar for any RUM barrel.

I'm sure the price has changed by now, seems like I paid around $168 for it. Then while I was sick and out of the house some sticky fingered bastard stole it and a bunch of other stuff.

George

It is not tricky; it is straightforward machining to make reamer pilots. I make them out of brass. I have RUM reamers.