Short of going to a gunsmith, what I have done to measure chamber length is take a dowel rod almost the size of the chamber. Wrap tape around it so it is snug, not super tight and insert it into the chamber until it stops when it hits the forcing cone. Mark the dowel at the face of the chamber and remove the rod and measure it. You can also cut an old credit card or the likes to the proper width of the chamber and insert it and measure it.