Consider this,
When you hear people say " pressure signs are not a valid way to check for pressure." you are hearing only half the truth.
When they say " pressure signs are not a valid way to check for pressure." and stop there, what they are leaving out is "they do not reliably tell you what the pressure is."
Now, consider this,
DOES IT MATTER???
And what I mean by this is, does it really matter what the actual pressure value is? in psi, cup, or what ever units are used?
What primer signs and all the other pressure signs are telling you is that something has changed from "normal". They can't tell you that your load that you expect to be 35Kpsi is actually 37Kpsi, or 43Kpsi, only that there is something different, and its probably higher than expected pressure.
Different enough to leave marks that usually aren't there. When that happens its time to stop and figure out why. And to be aware that what you're looking at is unique to your gun and the specific combination of components you are using.
Different guns can, and do react differently to the same amount of pressure, and what may be fine in one could be too hot in another, and still be within the "safe" published loads in the manuals.
Just as different guns can give different velocities they can also react differently to the pressures involved.
As an illustrative example, some years ago I loaded some pretty hot 125gr .357 Mag ammo. It was a load out of the Speer manual of the day, a bit under their listed max, and according to the book should have been in the 1500fps range from a 6" barrel.
Along with a friend who supplied a 6" S&W M19 and a Marlin 1894 carbine, I supplied a 6" S&W M28, and a 6" Desert Eagle. And a Chrony...
My friend fired his M19 first, firing over the Chrony and he doubled the gun (fired two shots instead of one). To this day, I don't know how he did it, and neither did he (he said...
)
Chrony said 1620fps and we decided to stop using the M19 at that point. Unfired rounds fell out, but the two fired cases, were stuck. Primers were a bit flattened, but not extremely so, but the cases were stuck in the cylinder and could not be ejected by hand pressure, having to be driven out with a rod and small hammer.
I then shot that ammo from my M28. Very stout. 6 rnds avg mv 1670fps. and cases ejected with hand pressure. primers looked like the ones fired from the M19. Slightly flattened. No other signs of excess pressure.
Fired the same ammo from the Desert Eagle, 9 rnds (full mag), MV avg was 1720fps. Function was flawless. Primers looked just like the ones from the other guns, and no other pressure signs on the cases.
Fired from the Marlin carbine, the results were the same, except MV from the carbine was 2200fps.
4 guns, shooting the same ammo, differnt MV with the 3 with the same nominal barrel length, but different effects on the cases in one of them.
At a later time, I fired some of the same ammo from my Contender (no chrony) and while the case extracted normally when the action was opened, I couldn't pull it out with my fingers.
SO, here we have an example of a load in the books, which, while safe (nothing blew up, or out) was too hot for a couple of the guns that fired it, but not for the others.
What I'm trying to get across here is that, my gun, or your gun, with our individual combinations of components, will do what ever they do, and that is what matters more than any arbitrary pressure limit expressed as a psi number.
Some guns & ammo combinations will show various pressure signs when the pressure is well below standard max working limits. Other guns won't show signs until well beyond the max working limits and possibly not until you're past working limits and into the actual safety boundary area.
Pressure signs won't tell you what you've got, but GETTING them tells you something isn't quite right with the load and gun you're shooting.
I am NOT advocating exceeding any published pressure limits. Just giving some points to ponder, for discussion.