I have a two head model that I purchased new from Brownell's years ago. It came with the tool, two PVX protectors and two blued steel calibration rings. One is .700 and one is .600. With those two it will measure from 28 ga to a standard 10 ga (barely). Chambers only go through 12. I tried the PVC tubes and they are not even close to calibration. After years of use, then only problem is that one of the heads tends to slip loose under use so I stick on a piece of tape. When I get a chance I intend to put a set screw through the brass mount to get rid of depending on the slip fit. The small dial on the face of the main indicator is important to keep in order to correctly read the tenths since the dial runs over several revelations across the gauges. I have never found a need to change or remove it, so don't know exactly how the dial is attached. If you elect to change it be sure you use a multirevolution dial indicator. Anyone with a lathe should be able to make a calibration gauge. The stock ones are more of a tube form; smaller in OD and longer. The longer form is much easier to use in practice. It could be made from any piece of appropriate tubing and bored to ID.