A few comments here. In early days of US shotguns "Standard" chamber lengths were 20& 28 gauges 2", 16 gauge 2 9/16", 12 gauge 2 5/8" & 10 gauge 2 5/8". Other lengths were available on order in some cases up to 3" length. Very soon 2 3/4" also became a standard chambering in 12 gauge & 2 7/8" in 10 gauge.
I have a 12 gauge G grade Lefever SN 20,633 which is a two hook Damascus gun which has 2 5/8" chambers. I also have a large single hook G grade Damascus SN 29,471 which has 2 3/4" chambers. An E grade 10 gauge Damascus of mine is a rod cocker with side pivot top lever SN 10,515 has 3" chambers.
My personal with older guns is one is much more apt to run into chambers a few thousandths undersize than oversize. Current SAAMI specs suggest a minimum chamber diameter at the junction with the cone of 0.798"
What effect does this have? "IF" the chamber is even .003" over the gage diameter & you have one of the newer 1" long forcing cones the gage will go in approximately 1/16" deep.
Chambers will normally have as a maximum a taper of .005" per inch. A gage even .001" larger than the end of the chamber will thus stop a whopping .200" short, showing a chamber more than 3/16" short. A chamber 2 or three thousandths undersize does NOT create a dangerous situation. I have little doubt that many shooters have been sold a re-chambering job when it was actually to length, to begin with, but a bit undersized.
I do not have a current modern chamber length gage to measure. "IOF" someone does & also has a micrometer I would like to have an actual measurement of the Working portion's diameter. I highly suspect it will be right on that 0.798" SAAMI Minimum. This is why working with older guns I find the 6" scale to actually in many cases the more accurate method.
Will also comment we use the term Machinists scale, and yes I do too Stan.
The scale doesn't have to be Machinist quality as in a Starrett or Browne & Sharpe Etc, a General from your local Home Improvement store will do the job quite well. We're not trying to measure down in Thousandths of an inch here.
