The idea of the 300RCM is similar to the idea of the 375 Ruger.
375 Ruger= 375 H&H (24" barrel) performance in 20" barrel, or slight increase in performance in their 23" barreled rifles.
300RCM = close to 300WSM (24" barrel) performance in a 20" barrel.
The 300RCM concept is sound- A 20 " barrel rifle is nice and compact, and since it shares a similar head diameter of any of the belted mags, it should be easy to accommodate in similar rifles, unlike the WSM, which is fatter and some people (not me) have complained about feeding, etc issues.
It uses special powder blends not available to handloaders, but most hunters and shooters dont' handload, so if a guy wants a compact rifle with pretty close performance ot the 300WSM or 300WM, it's a good option.
It is probably a more sensible cartridge in some respects than the WSM, but I think it's a bit too little, too late..I dont' personally expect it to sell extremely well, but I'd buy one for interests sakeand check it out if I didnt' have a very nice 300WSM that is essentially perfect.
The 338 RCM is another story, though, as there will not be a 338WSM factory offering. Could be a very good cartridge.
