What kept them from getting popular and staying that way?
I remember the articles from back then. All about velocity. The WSMs had the largest cases, so they were a little faster and that was what mattered at the time. After all, the Short Mags were some of the last of the "light bullet, fast and flat" cartridge designs. Many of the articles didn't even test for accuracy, just velocity. The holy grail was cramming it into a true (2.8"-ish) short action and going fast. I've linked to a 2010 Rifleshooter Mag article for the 300 short mags including the RCM, as an example, more fair than most. It points out the WSM's advantage with the light bullet, which was a best seller in factory ammo. Interestingly this article didn't test factory ammo which put the RCM at a disadvantage since Hornady had just developed superformance powders which made up for the RCM's powder volume deficit, especially out of compact, 20" barrels.
The WSMs maximized case volume largely by having slightly longer cases with short necks... but this left no room for freebore in the chamber, just a tapered throat. The SAUMs, arguably have the better design. The 7mm SAUM, for example have 0.09" of snug freebore (0.2846" dia) and a 0.31"-long neck which was preferred by those concerned more with accuracy than velocity. When put on a longer action, case capacity can be gained by loading long and bullets with longer ogives can be used, of course.
It looks that only the 300 WSM has remained somewhat popular as a factory cartridge. I suspect it was easier to tune for accuracy than the 300 Win Mag and its relatively sloppy chamber. Of course American hunters traditionally love .30 cal. 7mm has gained wider popularity in the past decade or so. For a long time, the Rem Mag was it. Many also believe that the 7mm Rem Mag had a better reputation for accuracy than the 300 WM so the 270 and 7mm short mags had a tougher time being seen as an improvement.
My guess is that there was also not enough difference between the 270 WSM, 7mm SAUM and 7mm SAUM for any one of them to become a clear winner. They each cannibalized each other's sales for rifles and ammo. The 270 WSM had a little more support from 270 die-hards, but the 6.8 Western has largely killed that.
How do the short mags from Winchester, Remington and Ruger stack up?
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