Rusanon, what do you think, how would a straight-walled 8,58mm magnum cartridge perform? In metric that's .338, so it's just between .327 Federal Magnum (or .30 Carbine) and .357 Magnum, but not halfway through.
>I think it'd probably be pointless, being so close to a lot of other revolver cartridges
I can see two angles to pursue here: a military and a revoler/lever rifle one. .338 Lapua Magnum is basically the de-facto sniper cartridge by now, and we might see it adapted as a machine gun cartridge in the future. So if you had a .338 pistol cartridge, then you could use the same barrel blanks, like how the Russians did it with all of their 7.62 cartridges. It's not important in the greater scheme, but it makes my autism tingle. After all, if you go for new cartridges, then you could might as well do this.
As for revolvers, a semi-rimmed version could work like federal magnum. That is, maybe you could cram six of them into a cylinder that can only hold 5 .357 Magnum cartridges. Yet it would be somehow bigger, so you could dispell some of the worries about the Federal Magnum's performance. And although I can't find definitive information about it, I suspect 5.56 NATO chargers (or stripper clips, if you prefer useless neologisms) should be able to hold .327 Federal Magnum the same way 7.62x39mm chargers hold .357 cartridges. Therefore, if our semi-rimmed .338 cartridge has the same rim diameter as the Federal, then you could use those chargers as speed loaders. In addition, if you make a lever gun for the longer version you've mentioned, then this could be a companion cartridge specifically for lighter loads.