Can I form 338 Win Mag cases from 7mm Rem Mag cases. I am worried that the large upsizing might stretch the brass in the case neck to much. Has anyone tried this before?

Well, going from .284 ID to .338 ID is quite a stretch. One of the problems is that brass tends to stretch unevenly when necked-up, so you may find thin spots or even splits in neck after going through this operation (or upon firing). If you want to try, be sure to use a tapered expander. Alternatively, you could use in succession a .30, 8mm, then .338 expander.

Some will suggest annealing the necks before you expand. I would advise that you try a few first to see if it works without annealing. Annealing must be done VERY carefully or you can make the brass so soft that it just collapses. Annealing AFTER expanding might make more sense, depending on the initial condition of the brass you are working with.

Good luck!

I've necked up 7mm Rem Mag to .358 Norma Mag without problems. A friend with a lathe made me a nice long tapered expander. I lost about 10 cases of the first 30 I reformed, but then zero of the next 80.

I had no problems with collapses when I necked up before or after annealing.

I didn't measure the runout before firing, but it was very good after firing so if the case necks were uneven then firing evened them out nicely.

In case you are wondering why I used 7mm Rem Mag cases rather than a calibre closer to .358: well I didn't have any of those and I had lots of 7mm cases!

I've been told that fireformign with cream of wheat instead of a bullet is better, but my method uses fewer primers!

jpb (not a newb to this forum, but my post counter got reset in the big crash)

I've reformed them easily. Only problem is the expander. A good tapered one, such as Hornady's, is necessary or you will occasionally collapse the shoulder. Amazingly there isn't much thinning of the neck. It's a simple conversion.

I do it in two steps:
first one is using a .30 calibre expander with the sizer die screwed out a bit, so there is no real sizing of the case;
second one is full length sizing using the normal .338 expander. Seems to work OK and haven't lost any cases yet.

Been there tried that. You end up with a short neck if you don't ruin it first.

I attempted to form 338 WM from 300 WM and could not do it, I think this was due to the fact that a 300's shoulder is about 1/4 inch longer than a 338.However I had a couple of 7MM mag laying around and they are of the same lenght as a 338 (2.5 inch) and was able to reform them. What I did was to heat the cases neck and shoulder with a propane torch till it was glowing then ran it thru the 338 sizing die.After checking dimensions I did load these rounds and fired it with no problem.

I just found it a lot easier to go buy my 338 mag cases. Somethings are just not worth the trouble to do.

I made 150 cases for my .375 taylow out of 7mm Rem. Mag. brass. I annealed the necks first, then ran them through the full length sizer with a tapered expander. I then trimmed them just enough to square the necks. Total lost cases out of the 150? Just five. Worked just fine for me. cases have lasted for five full power loads so far.
Paul B.