An internal spring caliper works very well, simply slide it down the chamber with a little spring tension on it. When it stops you've reached the end of the chamber. Next carefully mark with a felt tip where it protrudes even with the chamber opening. Finally screw the spring tension adjustment screw in slightly to remove spring tension and remove from the chamber. Measure the distance from the caliper end and the felt tip mark with a ruler for your chamber length.
One added bonus, one internal spring caliper can measure many different sized chambers, you can also use it, combined with a micrometer to measure choke and bore sizes.
Steve
