I make 350 Rem Mag brass out of other magnum calibers... 7mm Rem Mag, Remington brand is easy cut to correct COAL and size, but others work too. Redding makes a die just for this. It works great, I made a few hundred awhile back.
I have a new bag of 350 Rem mag and a box of Nosler 350 mag brass but they are not for sale. I neck turn all the brass ... but you have to with 300 WM cases as they are too thick, I've used every mag case that has the correct head size even 300 H&H, but they are now rare and expensive. I have a model 7 thats been altered to use 2.910" cartridges, piller bedded, tactical bolt knob, muzzle brake on the factory 20" barrel. 200 gr Barns 2940 fps, 225 Barns 2750fps, 250 gr 2626 fps, 280 gr Swift at 2450 fps 310 Woodleigh 2250 fps. An excellent cartridge accurate and plenty of power for most hunting endeavors...plus cheap plinking with bulk .357 pistol bullets or 160, 180 gr, .357 Legend bullets...a very versatile caliber.