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!?
The greater cross sectional area of the .358 should make a noticable difference in impact on big critter, especially at RUM velocities (most noticable <1k yds). Why isn't the .358 caliber more popular? Just as stated before, there are no viable long range bullets in said caliber. Would the UMT still kill elk @ 1kyds? Dern tootin'. Not to mention it should offer greater bore life than the 7mm, 300 & 338 RUMs respectively.
I think it would be a lot of fun personally, I've always tried to have the oddball cartridge in camp. That's why I chose the 7.82 Partriot over the 300WSM, that's why I chose the 6.5WSM, or the 22-243 over the 22-250 & the 6-284 over the .243 win.
In the end the question most often posed is: "Why?". The most simple answer is: "Because I can".
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