338 Federal Brass
As of now (August 2008), no one is selling new, properly headstamped unloaded brass. Federal obviously manufactures it for loaded cartridges, but does not sell it as a component. When asked, Federal reported that they would not sell new unprimed brass for this cartridge. Consequently, the only way to acquire brass for reloading is to fire factory ammunition or to reform another case.
Necking Up 308 Winchester Cases
Reforming brass is not a big deal. That's how 338-08 brass was made back in its wildcat days. No special forming dies are needed.
To form 338 Federal cases from 308 Winchester cases, just use your 338 Federal sizing die. The inside of the case neck should be well lubricated. Imperial Sizing Wax seems to the best recommendation for this job. After resizing, be sure to carefully inspect the case necks for splits. Also, take care not to mix up loaded ammunition, as the headstamp on the cases could be confusing.
Note that after I tried this, the resulting cartridges were difficult to chamber. It took a little effort to close the bolt on them. However, this did not happen with factory ammunition. Fire-forming the cases should take care of this.
Expanding the neck of the case will cause the neck walls to become thinner. This will decrease the useful life of case, and it will also decrease neck tension. Worse, the expansion can cause the neck walls to stretch non-uniformly, causing uneven neck wall thickness. This will contribute to bullet runout and decrease accuracy.
Necking Down 358 Winchester Cases
For precision handloads, it is better to neck down than to neck up. The neck walls become thicker, rather than thinner. The necks must be turned to bring them down to a thickness that will chamber properly, but this has the advantage of creating very uniform neck wall thickness.
However, the only tool I know about that can do this is the Redding Type-S Bushing dies. They allow you to insert bushings of various sizes to compress the neck down from the outside. Unfortunately, they do not make bushings large enough to accomodate 358 Winchester brass. The largest bushing appears to be .369", whereas the outside neck diameter of 358 Winchester brass is in the neighborhood of .386".
So as far as I know, necking down 358 Winchester brass is not currently an option.
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