With a bit of practice a 6" steel rule can be used to feel the start of the forcing cone and thus the length of the chamber.
That is how I do it.
Set the barrels in the vise, shine a light in the chamber. put the rule into chamber until you touch the forcing cone, read the rule at the outside edge of chamber.
O.M
Yes! This is easy, cheap, and accurate. You don't even need the flashlight or a vise to do it. Set the barrels on top of a cardboard box on a table. Sit in a chair and look down the barrels while they are pointed toward a window. You can see the end of the 6" steel rule as it reaches the beginning of the forcing cone. Done this way, you can then also learn what it feels like when you find the end of the chamber. But, it can be tough to feel the start of the forcing cone when the taper is long and the transition is gradual. That's why I find it best to actually look into the chamber to see where the end of the 6" machinist ruler is. As you can see from the diagram below, a 2 3/4" or 2.750" chamber is measured from the very breech end of the barrel.
