Per Mr. Researcher: "The chamber body itself has a taper of about .005" per inch. Sometimes chambers were cut with slightly worn reamers giving a slight undersize chamber. As you can readily see if the chamber should be .001" under gauge size at the end this would give a false length reading of about .200" with a gauge. The machinist’s scale method can be more accurate."
Mr Researcher, you have some fuzzy math there. Assuming a 2 3/4" chamber, the actual difference in length with a .001" undersize reamer would only be about .004" (too small to measure with a machinist's scale).
You can't just do a direct proportion .001/.005 = X/1.000. That incorrect formula would indeed give you the incorrect .200" difference in length you cited.
Using the figures in Dunlap's book for a 16 gauge, 2 3/4" chamber, the chamber diameter at 2 3/4" from the breech face is .732". Thus the correct calculation is .732/2.75 = .731/X. Solving for X will yield 2.746" or a .004" difference in length from a .001" undersize reamer.
Even if the reamer were .005" undersize the difference in length would only be about .020". Again too small to measure with a machinist scale.
The machinist scale method can still be used to measure length but be advised you are not looking for anything like a .200" delta with a .001" undersize reamer. fermat