Hi Monty,
You're definitely not stupid, just new to measuring barrels. There are several measurements that matter and you're just confusing them.
When you buy a gun, the seller typically publishes 3 measurements.
1. Choke Constriction (measuring bore diameter and muzzle diamater, subtracting the larger number from the smaller)
2. Minimum Wall Thickness (measuring the entire length of the gun's walls for the thinnest point)
3. Bore Diameter (measuring the nominal bore diameter to verify the gun is still in proof, e.g. .729 = 12 bore, .719 = 11 bore, .740 = 13 bore)
The confusing part is that minimum wall thickness is arguably less important than the measurement that is rarely published. The key measurement is 9" from the breech, does the wall thickness approximate .050" or greater? (the location where barrels like to explode and remove your hand in the process)
As indicated by other responses to your note, your measurement math is complex (but correct), yet it will mislead you. Barrels are usually bored cockeyed so that one side of the bore will be significantly different from the other side. You can't measure the outside at 9" with one tool, measure the inside at the same 9" with another tool, do some math and then get a true answer. Your results are misleading and do not account for concentricity issues of the barrel.
You need a $500 wall thickness tool to give you the actual measurements you truly need with any amount of reliable accuracy.
Imagine this scenario: There is a dent that was raised on your gun 12" from the breech of your gun and it was then honed from the inside and polished from the outside. It is invisible to your eyes from any location. Any tool short of a wall thickness gauge will omit that reading that actually shows the gun is only .014" in an area that is otherwise .038" at the same length from breech, but just at a different location along the circumference of the bore. Dangerous!
-Rookhawk
