I'm not sure about the pressures of 30-40 Krag vs. 35 Remington. The 35 Rem is a lot hotter than the Krag. I know the 8x54 Krag has a chamber pressure of 42Kpsi, and as I recall, .30US was less than 32Kpsi.
I may be wrong, but I think you're oversimplifying the problems that need to be considered in chossing a cartridge for this action. The distributed force over a smaller area increases the stress significantly. The 12ga forces are distributed over .62 sq in, but the 30US, and most similar 7mm, .30, and 8mm cases are less than .2 sq. in.
Also consider the action type; the center line distance of the SC, above the pivot point (Front lug) is close to 3/4" while most center fire cartridges are in line with a bolt face. The further away the center line deviates from the from the points restraining it, the more important a top locking lug becomes.
I love the 8x68S but I wouldn't subject it's 52,600 psi chamber pressure to an action design to restrain 12kpsi
This also does not take into account the rapid drop of of pressure is a shotshell vs distance from the chamber as opposed to the progressive burning powders where pressures are still increasing away from the chamber.
Do you recall seeing the stalking rifle in the rack at my table in Baltimore? It was a wonderful single shot in 9.3x72, and it had a single locking crossbolt, in addition to two locking underlugs. There must have been a reason for three locking points, above, and below the centerline of the cartridge.
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