Jack O'Connor said, decades before the current overboring craze began: "Trying to determine choke with a dime proves only that you have a dime in one hand and a barrel in the other". Key-ring brass gimmicks were useless than and they're more useless now. I carry a "snap gauge" in my bag to gun shows. Mine only goes in about 2-1/2" though and doesn't tell the whole story unless the choke taper is pretty short. I think you can buy a Skeets "shorty" model that's only about 6" long.
Choke measurements are really not the "Black Magic" mystery some of us want to believe. The standard dimensions will pretty much deliver the expected patterns. That's why they are used by every manufacturer and real gunsmith. Testing with quality shot/ammo at the pattern board is surely the ultimate proof but it rarely produces any real surprises. As to measuring the "entire bore", only the few inches leading up to the choke area are really important. Otherwise, jug choking would not work, and it certainly does.
