I'd get a quality digital caliper, unless you already have one. I got a Mitutoyo with carbide jaws. IT is my most used tool, Expensive but worth every penny.
For your web thickness measurement use the zero function. Zero (tare out) the barrel width. Lay a piece of 1/4" rod in the ramrod channel. Put the barrel n the stock, measure from the barrel across to the rod in the ramrod channel. Take away 1/4".
I use the caliper for depth measurements, inside, outside and as a scribe. I almost never use a micrometer.
I have a nice vernier caliper I never use. I can not see the markings very well anyway. It is like a slide rule, why go there if I have a calculator?
If you want to be historically accurate that is a different question.