This is one of the handiest tools you can have. I use it to measure choke constriction and chamber depth. I have the legs marked at 2 1/2" , 2 5/8", 2 3/4" and 3". Just put legs in the chamber and slide it in slowly until you see the needle move indicating the end of the chamber.
Also insert it all the way in the muzzle end and withdraw it to measure the choke constriction. I was accurate to within .001" vs my gunsmiths friends high dollar shotgun bore gauge.
https://www.amazon.com/DBM-IMPORTS-I...%2C220&sr=8-12
Threaded Mode