The issue isn't pressure, at least not directly, it is end thrust. 35,000 psi over the inside of that small case,
ignoring the friction of the case on the chamber walls, puts a normal force on the breech block
of about 3500 lbs. The Low Wall was factory chambered in .44-40. Guessing at the inside rear
case diam at .450, the area is 1.6 times the .357 case inside area. SAAMI pressures are CUP, so not
directly usable here. I decided that .45 ACP std pressure is probably about what PSI a .44-40 would
do. They could push a 200 gr bullet 1000 fps in a BP revolver, so that is about what
you can get with a 200 SWC in a .45 ACP at max pressure, so pressures are in the same ballpark. SAAMI
limit on .45 ACP is 21,000 psi and that gives a case thrust on .44-40 of just under 3200 lbs. So, with
full magnum loads, he may be giving it 10% more force on the breech block than .44-40. I don't think they
are anywhere near running within 10% of problems with the original cartridges, so it is probably
just fine for them some of the time. Maybe a steady diet of max magnums might be detrimental
in the long term, but probably not. I would bet that the safety factor is more like 2X normal
pressure requried to get an actual failure, likely more.
Two issues on pressure containment - rupturing the barrel, and bolt thrust. Of course the other
issue is how an action would deal with a failed case. In this caliber the only area of unsupported
brass is at the extractor, a small area of solid brass head. Any brass neck or body splitting would be
forward and entirely inside the chamber.
OK, so you say you have a .38-55 the rear case area there is around 1.32 times the .357, the only pressure
for SAAMI is 30,000 CUP. No real conversion, but if we work it out saying CUP=PSI (not really valid
but better than rolling dice, maybe) you get a breech block load of about 4,000 lbs. That is certainly
not a precise number but is at least in the ballpark.
Bill