The shortmags share the short n' fat powder column of the most successful benchrest cartridges (6mm PPC, 22 PPC, 6mm BR, etc.), a fact Winchester used to kind of imply that their WSM were somehow as accurate as the benchrest standards.
Of course, they don't share the benchrest-standard small flash holes and small primer pockets, and they use magnum primers, all of which pretty much takes them out of the running for anything the benchrest community considers 'real' accuracy. But, they look the same and it's good ad copy, so...
I've never heard them claim the shoulder angle had anything to do with accuracy, but they do claim that they're more accurate than 'standard' belted magnums because they headspace on the shoulder, not on the belt. Not only is this a long ways from accepted engineering fact, most belted magnums _don't actually headspace on the belt_. The only reason they have the belt is because the case designs were taken from the ol' H&H .375 (or really, the .300). The 7mm Rem, 338 Win, 264 Win, 300 Win, all headspace on the shoulder. So even in the unlikely event that shoulder was somehow more accurate than belt, it would be a non-issue because all the rest of the magnums already headspace on the shoulder.
The Rem Ultra Mags are ginormous, tall (i.e not 'short n' fat') sort of super magnums based on the .404 Jeffrey. They use a rebated rim AND a beltless case to maximize case capacity in a standard magnum-length action and bolt face. Nobody's claiming that they're more accurate, just that they're HUGE and very fast. Imagine seating a bullet right in the mouth of a 1 lb. can of Retumbo--that's pretty much the design principle behind the RUMs.