Up until now I've been using split ball bore gages to measure the effects of muzzle treatments and general small hole dimensions. The down side of these instruments is that they require a transfer measurement to get the numbers. This method will allow for only one static measurement at a time. This makes probing the bore at more than one place slow and tedious. Also, the radius of each 1/2 ball is generally too large to get a measurement down to the bottom of the grooves in the rifling so you're left with only the bore diameter at the lands.
I've just acquired a dial type bore gage that uses a different technique. It's a vintage Federal Products instrument that makes dynamic measurements as you continuously probe around in the private parts of a bore. It will reach in 2 1/2" and reads directly to 0.0001" It can easily be interpolated to 1/2 of a tenth. There are 12 probes that come with the set and each probe has a range of 0.008". The full range on this set is from 0.122" to 0.250". This will cover the barrel bore diameters of interest to me now. The use of setting rings is a big help in setting and calibrating the probes, but a good micrometer can also be used, thought it does make for an awkward measurement.
Besides the dynamic reading capability, I like the radius difference between the larger and longer fixed anvil and the smaller radius of the moving ball anvil. The ball can get down into the bottom of the grooves and the longer fixed anvil can straddle the grooves while staying on the faces of the lands. This allows accurate step height measurements of the rifling.
The photos show the basic arrangement of the instrument.