I also noticed this:
Keep in mind that the .375 Winchester is a straight walled case, and most of it's stress during the firing cycle is outward, against the case walls... not back against the bolt face! *(P.O. Ackley did exhaustive studies on this phenomonon and conclusively proved the point)
explains why straight walled pistol cases are fine on the SB1.
I remember reading Ackley, and about his experiments with a Win94 with the locking bolts removed and the gun opening on firing ONLY when pressure was a lot less than max.
Once pressures got up to standard operating pressures, a properly fitted brass in a clean chamber had little if any backthrust.
I wish I was rich, so I could test to destruction, a Handi SB1 in 44mag stretched out to SuperMag length, or more, loaded to 60,000psi