Buck Elliott wrote:"Bolt thrust" is... directly proportional to the gas pressure generated and the area of the case head supported by the bolt or standing breech.
If you have any solid, empirical evidence to back up that assertion, I'd like to know about it. Shooting a marginal rifle in a mid-power cartridge, I'm always on the look out for information on breech thrust, but hard data isn't growing on trees. As I said before, I really need to try out that method Sisk has used.
If by your statement you mean pressure times the ID area of the case, then what evidence I've come across says otherwise. The British base crusher system clearly demonstrates case cling is a big factor. As mentioned, oiling the case changes the indicated breech thrust by about 25%. Further, using crushers to measure breech thrust allows one to estimate case pressure, yet these estimates were 10 to 20 % below what using a crusher to measure the pressure radially via a hole in the case showed. Again, case cling is to be suspected.
The best single work on this matter I've come across is
Varmint Al's analysis of the effect of case cling in a .243 Win at mid and high pressures. It's a big page with much information to digest, but with a ruler and a calculator, one finds that over 1/5 of the breech thrust (as computed by case ID) is carried by the brass into the chamber walls. This is in line with the British numbers, as compared to radial crushers.
His analysis was done with a finite element system capable of estimating plastic strain. Numerical analyses are not perfect and can't be taken literally without backing by proper lab stress measurements; however, besides the British numbers, Al's analysis includes an estimate of case head expansion, and those numbers are in line with some measurements I've read about (I don't use the method myself).
In looking over Al's data, one sees a pitted chamber at 35,000 psi in principle can operate without the brass ever touching the breech yet stretching to the point head separation will occur with enough reloads. That doesn't mean there won't be any breech thrust. His estimates also indicate that once about 2000 psi is exceeded, the wee "piston" of the primer starts to exert significant thrust.
The thrust of the primer suggests that only the wimpiest loads on thinner brass in poor chambers might make zero thrust -- say a .22 Hornet firing a round ball with a puny charge. Otherwise, there will be enough thrust to start moving the breech.
Regarding the blowback pistols, Al's analysis gives no direct answers. The case he analyzed is long and has shoulders to push forward under pressure. Still, it shows the case base grinding rearward by the time 5 ksi is reached -- well below the operating pressures of such pistols. My guess is the typical, short blow back case moves back at lower pressures than that.
Sixgun's information on the Lightnings is also relevant, but I'm not sure what to make of it.