I have a Starrett telescoping gage with about a 12" handle. For as far as it will reach into a barrel I can easily read the diameter to within .001" with no problem.
As I recall (Don't own one) the Skeets gage is a dial bore gage. Used them many times in my years as a machinist. Could certainly get a reading within .0005" with one of those. You have to set them with a ring gage but the bore contacts are diametrically opposed in a dial bore gage. A solid bar connects to the dial indicator. If it won't read closer than .003" then its total junk, best to throw it away & don't depend on any reading you take with it.
An internal micrometer works on essentially the same principal, except you're reading a micrometer thimble rather than a dial. The still have a solid bar going down the length of the shank many of these have three contact points which allows for better checking of out of roundness, but not greater accuracy.
