I'm wondering looking at all of this (and I should admit I only understood about a third of it) is there wouldn't be some merit in what I'll call a dual-friction barrel.
Imagine that the first couple of inches of the breach end of the barrel is high-friction, then leading in to low friction. Surely this would allow a higher pressure to build in the chamber during combustion, and thus more gas to be combusted, before the spud makes any initial movement, but also allow the spud to move freely once this high rate of combustion has been acheived.
I'd be interested to see for instance if there was any difference in performance between barrels with internal and external chamferings of the spud knife.
Effectively what you're trying to reproduce is the "burst-disc" effect, but without a burst disc.
Secondary to this, what conclusions can we draw from this on hybrid performance in relation to disc burst pressures, can we model an optimal pressure (as a ratio of peak chamber pressure) for the disc to burst at.