The issue of a rebated rim is something troubling that I know little about, as I have no rifle for such a cartridge. I would have thought the whole thing would have been based on the ancient and reliable belted H&H magnum case if I had been making predictions on this. I guess the 404 Jeffery case lets them equal the performance of the old belted magnums in the shorter case.
As for the idea of an extra short magnum, I think it is a natural. After all, the only reason the 300 Win Mag was invented was to take advantage of the 30/06 length receivers that all the riflemakers, with Winchester in the lead, were making at the time. That is the only advantage that it has over the 300 H&H and Weatherby Magnums that require a true magnum length action. This lead to the developement of a whole family of "short magnums": 458 WM, 338 WM, 7mm Rem. Mag. that proved very popular, much more so than the full length magnums, mainly because the rifles for the full length magnums cost more.
Now that the 308 Winchester is the standard military round and manufacturers are belatedy making short actions just for it, it makes extra short magnums aluring from the point of view of rifle manufacturers. They can put these more powerful cartridges to work in their new standard (?) actions.
Just because the action is shorter does not mean you have to build a light rifle out of it. You can make an even lighter light rifle if you want, but if you want a heavier rifle you can put more weight in the barrel were it will no doubt do more good than in the action. Yes, the shorter, fatter cartidge should have more consistant ignition and thus be more accurate, at least in fuzzy common wisdom. For bolt guns, the bolt throw should be a little shorter, always better. And most important from the perspective of manufacturers, the short actions can be produced in greater quantity and thus more efficiently.
Finally, as soon as the new case hits the market, you can bet that wildcatters will neck it up and down and improve it in every conceivable way, so that a whole family of new extra short magnums will be born. Winchester and the rest can just sit back and pick and choose among the most popular of the lot for their next hopefully popular introduction.
I'd say that an extra short magnum in the ever popular 308 caliber has been something begging to be born as soon as the shorter actions came on line. Actually, it is the flexibity and economy of computer assisted machining that has made these actions feasible in the market place, so I guess you can ultimately blame the whole thing on computers if you are not happy with it.
