It seems that 7MM rem mag brass would be suitable for forming .30-338 brass. Any comments are welcome.

I find it easier to got down instead of up. Run a 338WM in the die and be done with it.

Granted, I would rather neck down than up, if; you hve lots of 7 mag brass, by all means use it.

It is important to have a tapered expander plug though, and use some graphite to lube the inside of the neck.

Have you ever had problems with case neck thickness with sizeing brass down?

As long as you're sizing just the neck, no. One caliber in the area of 284, 308 and 338 usually translates to a thickening of about .0007" in my experience, hardly enough to concern yourself about in all but tight necked chambers. Where you might have problems is where you move the neck of the new case into the body of the parent case, such as shortening long cases in a WSM or something similar.

I prefer necking up 7mm Mag brass to necking down 338 Brass, since I have run into some thicker necks in my 30-338. Since changing over to 7mm Brass, I have had no need to neck turn anything. Ran into a thick neck problem with my 300 Savage when making 308 Brass into 300 Savage. So tight that a fired case would not admit a new bullet. Had to turn all of those to make them work. Brass was all Federal Match. Regards, Eagleye.