If you compare the round to the 375 H&H, you have essentially the same case, but rimless. You'll also see body taper is essentially identicle, but the Rigby has a 45 deg shoulder, so sharp enough and large enough to headspace off of. I can't help but think the 375 H&H was developed off of the same case, though stretched slightly to 2.85", and the belt being added to provide headspace, and also the shoulder being changed to a gradual taper.
The downsides of the round are the only brass available is Bertram, which is rediculously exspensive at $65-75 for 20 cases. All is not lost, as 375 H&H brass is de-belted and the rim reduced rather easily in a lathe, a run through the fl sizer kicks the shoulder back, and then a trim to length. I've even managed to squeeze a 340 Weatherby case down, certainly a bit more pressure on the press handle 
Case capacity is slightly more then a 358 Norma mag. My plan is to load mine with 250 gr a-frames @ 2700 fps from a 24" barrel. I will be re-chambering a 35 whelen ackley, and a .358" bore barrel with 1-12 or 1-14 twist makes the most sense to me for anyone considering building one, suite the barrel to the bullets available in this day and age. Reloading dies are avaialble from CH4D for $85 as I recall, std belted mag shellholder works fine.
One might also think, so what type of action do you use to harness such a round? If you use a 375 H&H type action, you have the length, but the bolt face is too big. If you use an -06, the bottom metal will be off, and you have to open up the bolt. I'm using a VZ-24, with bullets seated to the canalure, I'm comfortable with the amount of metal left behind the bottom lug of the action, the extra ~.10" compared to a 375 H&H round seems to be just enough. It looks like the lengthened magazene will hold 3 rounds.
If I were starting with a new action, I'm not sure which one is the best for the job. If anyone else is considering the project, feel free to e-mail me, or post here. There isn't much printed info on the round. I gleaned some from Cartridges of the World, but it's limited. My thought on load data is, with a case slightly larger then the 358 Norma, I'll just use Norma data, which is relatively common. I'm debating trying a 225 gr partition or X @ ~3000 fps, though figure the 250 a-frame @ 2700 will suite my hunting better.