I was not there. The barrel may have been short only .003 ,or three thousandths. That,IMO,would be uncommon.
What is far more common,no offense to the OP,is confusion about decimal points.
About .030,or thirty thousandths,would fit more with what I would expect in a short chambered barrel.
Remember to do the math and understand you are working with a shallow taper.Look at the "rise over run" idea.
If you had a case that was 2 in even,from bolt face to shoulder,and if your case taper was .040,or forty thousandths,thats .020 per inch on the dia,or .010per inch per side.
or .001 per .100 . So,if you could do a perfect job of recutting a chamber,you would need one tenth inch setback to clean a one thousandth flaw.(Your case taper will be different,this is for illustration)
Now,taking an impression,I used to use Co-Flex dental impression compound.A very "liquid" two part rubbery stuff that would flow into the finest detail.I could look at it under a microscope or on an optical comparator and see what I had,dimensionally.
You may not be able to do that,but here is something else.Silly Putty.You may be able to find it under "toys" try Amazon .
Silly putty will pick up the finest detail. Its temporary,the stuff is always flowing,but it can give you a look at what you have.