This is my take-off of the Galazan gage. I can measure 10 and 12 ga. on one rod and 16 and 20 ga. on the other. In 12 ga. with very tight choke around .690 I cannot put it muzzle down as it will not fit over the rod with the ball. I can measure 15 1/2" from breech end.
I put a level on mine mainly for 10 ga heavy wall barrels in doing muzzle end with all the weight now up, especially in the first few readings.
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In my opinion, to use the Hosford type gage, you really have to know what you are doing in measureing the breech end because of the weight on the bar. It would have to be measured more than once to get the right feel.
If I wanted to change the digital dial out to a Starrett, I could use a variety of tips and one is a needle tip if I wanted to get that critical next to both ribs.
I don't know how reliable a micrometer would be on the outside in combination with a dial bore gage.
I forgot to mention on doing 10 ga barrels with the muzzle down. The aluminum gage in the box is a 10 ga chamber gage with a spirit level mounted in it. When the base is level and the barrels are mounted and the muzzle end is in the tapered collar (brass) then I insert the chamber spirit level and then measure from top rib to bottom rib usually in 3-4" increments. Takes a while but then I know what I have.