It talks about it in the new Hunter magazine. I went over to my fathers today and it was laying on the coffee table. The thing that caught my eye was that it's falter than a 7mm mag. and faster than a 300 win. mag.
Knowing that this post is ~4yrs old, I can't help but comment as the .358UMT has intrigued me since Bryce wrote about it all those years ago.
Is it a niche cartridge? Yep. But one has to admit it is still pretty bad ***. Why would one build it? Probably for the same reason anyone uses to build a wildcat, because they want to & they can. I built a .358STA some years ago, 225 Barnes @ 3000 fps... why not!?
In the end the question most often posed is: "Why?". The most simple answer is: "Because I can".
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Outlaw, I'm stealing your idea, I'm now calling my .358 Ultra Mag imp. a "Because I can" Ultra Mag. Hope that's okay