An excellent alternative to pin gauges is a set of small hole adjustable inside gauges. Starrett and Mitutoyo make great ones. They have a split ball head that adjusts with a knurled knob on the end of the tool shaft. You insert it in the hole, adjust the ball until it touches opposite sides of the hole, then measure the ball with a caliper or micrometer.
These tools offer a few advantages. For one, they can detect an out-of-round hole. Also, you can carry these in your pocket to gun shows to check out potential purchases. This, and a set of feeler gauges for checking endshake and b/c gap, are very useful gun show tools!