I have been trying 458 Win Mag Brass. So far, it looks like the best, with no trimming required, and the shoulder in the right place after sizing.

I recommend you listen carefully to what John Ricks had to say.

I use 358 Norma brass, but for the reverse of what you are doing. I use it to size down to 338 Win Mag. The body of the Norma is .040 LONGER than 338 brass. I use it to reduce the significant stretching of the body that you get from factory 338 brass. This eliminates the initial stretch on the first shot and gives extend life to the brass for reloading. (I'm able to get excess of 20 reloads using Norma brass so sized and used in 338 Win Mag.)

If you will measure your body against the chamber of most rifles you'll find that the shoulder is short of the shoulder in the gun by anywhere from .014" up to .030". That's a lot of strech, and the reason for early case head separation in reloading the stuff.

If you form 338 brass to 358, you not only have that short shoulder distance in 338's but you have an ADDITIONAL .040" of stretch. That's a minimum of .054" that such a case is short of the shoulder. While it does headspace on the belt, you run the risk of separation on the very first shot. Certainly at the very least your number of reloads would be limited. You're treading on thin ice.

I was making brass out of 338 Win. and it worked just fine!!! then at a gun show I found a guy with 10 boxes of Norma 358 brass, WELL at $10 a box I figured that I would take all 10. So by taking all that he had he gave them to me at $8.00 a box!! wish he had more, none were shot.

I had bought my 358 Norma brass from Grafs. Then last year they had a close out and I bought 5 boxes of 250 gr Woodleigh (sp) for about $23 A BOX. I could buy the components for that. I wanted to keep the head stamp correct since I had a 338 and was already necking down 458 to make 416 Taylor.

Here's another option for you, and the way I went when I had a 358 Norma built about a year ago. I had the neck reamed out to a case length of 2.61". This was accomplished after standard chambering with the 358 Norma reamer, by then running in a 35 Whelen reamer with it's longer case neck, to the 2.61" chamber length. Why?? This lets me use untrimmed WW 300 Win brass. Just a pass through the FL sizer with well lubed case necks, and the brass is ready to go.

I use standard FL dies, in my case Redding, which work fine. The sizer die works exactly the same way, although I had the expander ball tapered more gently to make it easier to neck up the cases. To set the die I simply kept screwing the die into the press bit by bit, until the 300 cases had the shoulder pushed back far enough to fit the chamber. This works fine for resizing the fired cases as well, of course.

The seater die has to be backed off a bit, and the bullet seating stem screwed in farther to compensate. Otherwise it would crimp the case neck......a lot.

I have picked up a bit of case capacity, although I haven't really played with them to see how much. My thoughts with this case are to keep adding IMR4350 until the chrony reads 2800 fps with 250 grain North Forks, then go shoot something. Although I'm sure I could pick up a few fps, I'm just not sure what I'd really gain. I'm as guilty as the next guy when it comes to chronograph races with my handloads, but this rifle is designed to thump moose and elk. An extra 50 fps wouldn't really contribute anything.

My motivation to do this was to use the 300 Win brass with the longer shoulder. A lot of people use the 338 Win brass, but we all know the shoulder is shorter on the new 338 brass. I didn't like the idea of fireforming brass by blowing the shoulder forward, hence weakening the brass. It made more sense to push a shoulder back, maintaining positive headspace while fireforming. So I thought, seriously, about using WW 300 Win brass, then trimming about 1/10" off the resized case, which made no sense. Then I hit upon having the neck extended so I wouldn't have to trim the hell out of the new brass. It makes for a better case design.

It just makes more sense. With the longer neck, more of that long bullet is held under tension, just improving on the original design. It lets me use the inexpensive, great quality WW 300 Win brass, instead of buying the much more expensive, and historically soft Norma brass.

This was my only complaint with the original 358 Norma case design, in a mag action with room to spare, why saddle it with that short neck? The new case can be described as a 358 Norma case with a 35 Whelen neck, really the best of both worlds. If you do have this done, all it takes is running a 35 Whelen chamber reamer into your 358 Norma chamber. The shoulder/case aren't affected, just the neck is lengthened. Just be sure the neck dimensions of the reamers are similar, Norma neck dimensions are notorious for being on the "large" side. My chamber has a 2.6" "trim-to" length, with a max length of 2.61". This lets me trim the newly sized-up cases just enough to square the case mouths, instead of spending hours trimming.

This also lets you use 358 Norma factory rounds, it's essentially a freebore with those. It's such an obvious improvement to the standard 358 Norma case I can't understand why everybody doesn't do it!