OK, I gotta ask this - what does this new Ruger cartridge do that is different from the 284 Win cartridge? Capacity about the same as an '06 in a short action cartridge. Is there something else here that I am missing, besides the proprietary powder/loading of Hornady?
1) A non-rebated rim (if you care about that kind of thing).
2) This isn't in reply to your post, but compared to the WSM's, potentially one more round of mag capacity and smoother feeding.
From early appearances, it looks like the 300 RCM would allow for a 30-06 +P out of a short action with a relatively short barrel - that'd be a good thing. It's too early to tell, though, so I'd at least give the round a chance to prove (or disprove) itself.
