There is a book called Shotgun Gunsmithing by Ralph Walker that is pretty good. Brownells has it for $16.95. Nittler Gunsmithing Techniques, by Clarke, at $69.95 is good in a lot of shotgun areas. And there are general Gunsmithing books, like Gunsmith Kinks, that provide a lot of info on shotguns.
I am not sure what type of caliper you mean. There are 12" vernier calipers but they are pretty uncommon and of limited use. Let me know a little more about what you are interested in and maybe I can help.
Jim
