The brass base may or may not give you the right size for a chamber gage. "Nominal" dia of small end of chamber is; 10ga =.841", 10ga mag =.838; 12ga =.798", 12ga mag =.797"; 16ga =.732"; 20ga =.685"; 28ga =.614" & .410 =.463".
Realize that the chamber body itself has a taper of .005" per inch so if a chamber happens to be only .001 undersize (not uncommon on older guns) the gage will show it at about .200" short. Thus if turning a gage I prefer to make mine about .002" below those nominal minimums. It may go a short distance into the forcing cone, but that is of no consequence.
Generally I just pick up my old 6" machinist scale & hold the bbl up towrd a light source, throwing the forcing cone to appear as a shadow. I slide the scale in beside my face watching till the gap closes between the scale end & shadow line & slide my thumb nail up against the breech, for reading. After a little practice this can be done with plenty acceptable accuracy, & is quite inexpensive. You can even pick up a usable 6" flexible scale at most any home supply store, doesn't have to be a Starrett or Brown & Sharp. They are about 1/2" wide though ao will have to find somethig else for the .410.
