Just for grins I thought I would post the device I use to measure choke in 20, 16 and 12 gauge barrels. It is a Frank Mittermeier Universal Choke Gauge probably from the middle 60's. I inherited this gauge from my father and I have found it to produce reproducible results corresponding to known choke in my guns.

An image of this device can be viewed at https://www.ebay.com/itm/1874566750...38722076&customid=&toolid=10050.

enjoy

jlb