There's one other (rather indirect) index of chamber pressure that is worth mentioning. It is MUCH more of a "go no further!" indicator than a "safe to proceed" sign. Slide velocity, as measured by the distance cases get thrown, can tell the reloader that backing off is a good move. It depends on having a pistol that throws its brass more horizontally than vertically, but any pistol that ejects at a positive and acute angle to level ground can be calculated.
A reload that throws its brass a shorter distance than the brass of an established factory load (same bullet weight, etc.) and chronographs under the factory load's velocity is probably safe to use and work up higher. A reload that approximates factory in terms of ejection and projectile velocity may be a good place to stop. A reload that flings brass further, whatever the velocity, is a sign that things could become unsafe, at higher charge weights.
I HAVE encountered factory and lower-than-factory velocities with greater ejection distances, and decided that it was a "go no further" sign (primers were unremarkable). I've also obtained higher velocities (congruent with the charge weight vs. velocity "line") with shorter ejection distances and would have felt safe in going 0.1 gr. (or more) higher, but in most cases, I was satisfied with the velocity obtained.
It's a ROUGH indicator, and won't ever replace a piezo-electric transducer, but it might help keep someone out of trouble.
