Definitely not getting the 6.8spc into a 1.6" magazine. And I'm not sure we can even remotely approach that level of performance.
Building the cartridge on a whiteboard is actually pretty simple. We simply need a case fat enough that it can fit enough powder when short enough to still get ~35grn H2O capacity in a 1.0-1.1" case, and enough bore capacity to get up over 2000fps in whatever length barrel you're seeking. I'd rather have a larger bore, in that case, offering a heavier bullet for increased impact downrange, and I'll concede that a smaller bore than my 357/44mag might make sense, landing somewhere around 7mm to 30 cal - but we start losing powder capacity and bore efficiency as we neck down more and more. So something in a standard .473" bolt face should have plenty of capacity, then squish the shoulder back on something like the 7mm BR to be a 7mm Super Short, find some stubby little short ogive, flat based bullets in the 100-130 class and we'd likely get >2300fps out of a 1.6" OAL, still have enough bullet to do work on the business end, whether from a 10" barrel PDW or 16" rifle.
But then things get pretty weird on the other end - the firearm we have to build around the cartridge. When you've mentioned the extreme pressure standard used in the two/three piece 6.8x51/277 Fury, things get even more complicated. I'll admit, I don't look too closely at this, but I can't think of any grip-borne magazine repeaters which are wrapped around cartridges with 55,000-80,000psi pressure standards - for good reason. The 5.7x28 we've discussed is as close as I can recall, and we've discussed the design concessions which had to be made to make that function properly (special case coatings, delayed blowback & dynamic barrel mounting). We see the 44mag and 50AE Desert Eagle being gas operated. So, compared to conventional PDW type actions like the blowback CMR30, we know - in my best trolling boat captain voice - "we're gonna need a bigger boat." Like I said before, getting someone to design a gas operated action which COULD do the job might be easier than designing and manufacturing magazines for this...
I can tell you - the performance we're talking about is REALLY close to what I chased for ~20yrs before landing on my 357/44 B&D. Granted, I do have bigger game in mind than personal defense, and I'm working within the bounds of "customization" rather than pure "fabrication," but I played with 30-30 and 7-30 waters in specialty pistols which also approached these performance standards when pressed to Encore tolerances, rather then Contender limits. I did a "22 Hondo" which was a 218 Bee, shortened to allow 50grn bullets to fit into Ruger Single 6 cylinders, which obviously gave up a lot of performance to what we're discussing here, since it's only a .349" web dia, rather than the .473" we could be using...
It's a fun idea, as long as the realities are accepted - it's not a major market opportunity, and it gives up a LOT of performance to swallow the subjective criteria of a grip-borne magazine and the subsequently short 1.6" OAL. If we want super short lengths, let's talk bullpups - putting the magazine behind the grip allows even shorter length and commensurately longer barrel lengths for improved performance (and these still aren't popular), with not so dissimilar manual of arms to load/reload. But the world just started getting excited about mini-length cartridges about 20yrs ago, and that only to fit into a well established firearm design with ~40yrs of war under its belt - I'm not terribly certain we're ready to get excited about microlength cartridges.
Heck, the fact the Keltec CMR30 is alone in market is kind of indicative of market interest.
