Good info about slugging the chambers. Yes, the groove diameter should be the same as the chamber diameter within +0/-0.002". With the exception of certain premium priced guns that will not be the case. The chambers can be reamed to achieve that condition, however.
As for slugging the bore and measuring the slug: drive the slug at least one full ball diameter below the opening; you can drive it all the way through as well.
If the bore has an even number of lands and grooves (that is, the number of lands equals the number of grooves), the lands will be opposite each other and the grooves will be opposite each other, and you can directly measure the land-to-land diameter and the groove-to-groove diameter.
If the bore has an uneven number of lands and grooves (that is, there are more grooves than lands or vice versa) the measurement is much more complicated as the lands and grooves are opposite each other. You must essentially determine the land-to-land diameter using one slug (more about how to do this later), then use a second slug to get a measurement of the diameter of the slug from the land on one side to the opposite groove on the other side.
To get the land-to-land measurement, insert a soft lead slug in the bore which has a larger diameter than the bore. It will be necessary to rotate the slug, essentially scraping the impression of the grooves off the slug so that it is smooth. One way of doing this is to shape a slug such that it has a 'handle' that protrudes out from the bore that you can grasp with a pliers to turn the slug. Another method is to embed a tool in the slug to act as a handle before driving the slug into the bore. The resulting slug will be the same diameter as the land-to-land diameter of the bore.
Subtract the land-to-land diameter from the land-to-groove measurement determined from the first slug; the result is the groove depth. Add this groove depth to the land-to-groove diameter to get the groove-to-groove diameter.
