Steve, if you're thinking of using this information to "push the limits", there's a much more reliable, safer (relatively) and fairly inexpensive way to do it than eyeballs and micrometers.
No not at all. What prompted this question was a discussion I had with a fellow reloader about "pressure signs".
He is loading for his 223 and loading it hot, very hot. He tried to tell me that he is still within the MAP of 55K because there were no "pressure signs". He is convinced that "pressure signs" will show up as soon as chamber pressures exceed SAAMI MAP, in the 223 that would be just over 55K.
With my limited knowledge of chamber pressure I tried to tell him that the chamber and the brass do not know what SAAMI MAP is and that most pressure signs do not appear until well above 65K no matter the cartridge. But, I could not convince him as I have no real proof of this.