A 6" flexible machinist's scale is quite adequate for measuring the length of a chamber, even one of the inexpensive ones from your home improvement store will do fine. It is in some few cases even more accurate than a chamber gauge. Simply hold the bbls up toward a light, I normally use a window, & look in. move the bbls around till you have the cone thrown in a shadow. Slide the scale in where you can view it & watch for it to come flush with the shadow line. Do it a few times to ensure you are being consistent. With only a few tries you will find you can easily read the depth to within the 1/64" markings of the scale, plenty close enough for measuring chamber depth.
Should the chamber be a bit undersize, not uncommon on older guns, & be only .001" smaller at the end than the diameter of the gage, the gauge will show it about .200" short, which it isn't. The case where the scale is actually more accurate.
An old saying from my youth, "There's always more than one way to Skin a Cat", there are simply not many "Only Ways" to do anything.
