Well, that's not strictly true; I have a couple of rifles with 11mm bores, a 10.4mm, and a .577-450, but those are all black powder cartridge guns. The .405 Winchester T/C Encore is, however, my first big-bore smokeless cartridge rifle.
First released in 1904, the .405 Winchester was designed for, and remains romantically tied to, the Winchester Model 1895 lever-action rifle. Despite a moment of glory in Teddy Roosevelt's African adventures, the cartridge never really caught on with American shooters. Punishing recoil in the old-fashioned '95 with its crescent butt plate, plus a rainbow trajectory, meant that it just wasn't the gun for North American big game hunting.
With the re-introduction of the 1895, the .405 Win came back in a wave of nostalgia, currently being loaded by Hornady in both a 300gr softpoint and a 300gr Interlock spire-point (shown at right with a .223, a .308, and a .30-'06 for scale.) The Hornady load throws the 300gr .411" bullet at 2200 feet per second, which gives a satifying 3,220 ft-lb thump. Though a little short on sectional density for elephants, rhinos, and Cape Buff, it's still an ideal stopper on hogs and black bear, which I'm far more likely to have the chance to actually get to hunt here in Tennessee. If Jumbo escapes from the zoo, I'll just need to get a .416 Rigby barrel for the Encore. :)