Years ago I made one that I believe Miller (RIP) posted. His hung from the ceiling on a hook. The only problem with it was that you needed someone to write the readings down, or else you had to remove the barrels , write down your findings, and then start over where you left off. I got tired of that and was talking to a friend who had a Galazan thickness gauge. This one had 3 different bars, one for 10-12 ga, 1 for 16 ga. The hardest part was milling a tapered slot where the spring and lever part was flush with the rod. I don't have a milling machine and had to set the bar/s up on an angle and use an XY table on my drill press.I can read from the breech 16 inches. The breech end fits on a tapered cone that fits into the chamber and is adjustable with a thumb screw for different lengths along the barrel. In reading from the muzzle end, if the chokes are too tight I cannot go over the rod which is 5/8" (.625) and half the diameter of the ball .0625. Together that is .6875. So I can read at least 16 inches and really beyond that, most pressures I shoot under 7500 psi, so I'm not worried too much.
I can redo the readings, and they come out within a .00l or less, plus I can write everything down because the barrels are centered on the cone and the spring lever is holding them in place at that point.
Keith is right, nothing within our means can measure between the top and bottom ribs.
https://imgur.com/CRSvRu6https://imgur.com/67vgW5Ihttps://imgur.com/wD9V0unThe last photo shows a reading of .057, 8 inches from the breech.
