I have one of the Enco finger gauges mentioned above. It works fine on most Parker chokes but is limited to around 3". A few years back Dave Suponski offered Starrett inside diameter vernier calipers for sale here. They can measure to a depth of a bit over 5.5" and are accurate to .0001", I think. I couldn't get one fast enough. Not sure how large a diameter they'll measure but the smallest seems to be ~9/16"(.5625") so they're great for smaller bore guns of greater than .410. I'm not sure I've tried on on a 12ga or larger. Dave could likely give a better description. I do know I love this thing.
Looks like it actually can read to .001". Not sure. Dave, where are you?? Yes, it's .001" grads and has an official range of .575" - .750". I think you could measure a bit under the lower limit closely enough for choke measurements. This is not a gauge that Starrett currently offers.
BTW: I got my PP yesterday. I was reading "my" article and concluded that the next time I need a resume, I'm getting Mike to write it!
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