I've been reloading for years, but have never done any caseforming. Recently, I purchased a 7mm Remington Magnum Sigsauer rifle. I have 200 new nickel cases for the 300 Winchester magnum. Can the Win mags be sized to form 7mm Rem mags? Thanks, Stoney

Stoney, it could but being nickel, I would just trade it for some 7mag brass. I bet you'll find someone interested.
I've necked nickel 35whelen down to .338-06, but you would be moving the shoulder back & I'm not sure what effect that would have on the nickel plate.

I don�t know how it will turn out with plated brass, but reformning .300 WM to 7RM is not all that complicated. I use winchester brass (or Norma .308 NM brass preferably), I tried remington brass and it was ok, but needed annealing before reforming.

I just run the .300 WM brass trough my 7RM (forster) full size die, lubricate very sparingly (very, very sparingly, just so they don�t get stuck..) or you will have all kinds of buckles and creases in your newly formed brass. Then I trim them back with a Lee set up (my wilson trimmer is better but the lee thingie is fast and good enough at this stage), after that I prep the cases (turn the necks, although the win. brass is thin enough to begin with, so I just want to even them out a little, ream flasholes etc.)

I adjust the sizer so the new brass headspace snugly, but be sure to size them enough because cases vary in springiness so check that hey all go in you rifle...

With plated brass I guess there are no extra steps, but they can be very rough on you dies and I honestly don�t think you can/should/would turn the necks if needed. Trade them with unplated .300WM cases and make your self some excellent 7RM brass...

Tron

I don't think it is to your advantage to try to reform any plated case.

Trade the 300 for some 7mm, sell the 300 and buy 7mm, or just give the 300 away...you'll make a friend if nothing else.

I had no trouble at all necking-down some .308 nickeled brass to .243, but in that operation you are only reducing the neck and not displacing the shoulder. I think that the advice you have received to sell or trade is sound. If nothing else, you can put your brass on eBay or here on the AR classified section and someone will be happy to have it.