SHOTGUN BORE MICROMETER
You can’t be sure what choke you are using unless you measure it with a Bore Micrometer. The actual choke of a shotgun barrel is the difference between the largest diameter in the bore and the smallest diameter at the muzzle. The Shotgun Performance LLC Bore Micrometer is the perfect tool to determine what the actual choke of a shotgun barrel is.
 
Capable of measuring any 12 gauge bore and choke, as well as 10 gauge and 16 gauge, this tool is essential for any serious shotgunner, shooting coach, or gun shop. All critical components of the Bore Micrometer are machined on a Swiss Turning Machine to ensure precise measurements.

INSTRUCTIONS FOR USE
Simply turn on the digital indicator and insert the brass end into the aluminum collar just far enough for the steel balls to go into the larger diameter portion of the collar.
 
You may need to push the button on top of the indicator to retract the balls while inserting them in the collar. Before pushing “zero” make sure you have your finger off the button. Make sure the indicator reads zero with the collar over the steel balls. When the collar is removed it will read a number that doesn’t matter.
 
ENSURE THE SHOTGUN IS UNLOADED WITH THE ACTION OPEN!! Push the button on top of the indicator and insert the brass end of the micrometer in the barrel of the shotgun going in as far as necessary to get the largest reading on indicator. Add this number to .700 and that is the bore diameter. Pull the indicator out until the you get the smallest reading which should be at or very near the muzzle. Add this number to .700 and this is the choke diameter. The reading at the muzzle may be negative and this is correct. Subtract the smaller diameter from the larger diameter and you have your choke constriction in thousandths of an inch. See chart.