Ben, you can't use those internal calipers to measure the amount of constriction in a .410 because the steel arms are just too wide to allow enough reach into the bore. Bore diameter is irrelevant, choke diameter is irrelevant. What you need to measure is the difference between the bore ID and the choke ID. Nice to know choke taper, but you can't get it with that tool.
If you are willing to relegate that tool to a somewhat fragile status, you can try filing down the wide part of the probes so that you can get more reach into the bore. Max depth on that tool is 3.25" under the best of circumstances.
