Ultimate Arms Gear Gunsmith/Armorer's Bore Gauge Muzzle Loading Measuring Tool with Key Ring Hole Pocket Sized Measure in Calibers and Gauges Shows The Inside Diameter of Any Barrel from .22 Caliber to 10 Gauge. 3-1/4" Long With A Key Ring Hole. Marked in Inches (.22"-.80") Millimeters (5.5-11mm) and Gauge (.410 - 10). Punched for a lanyard, this nice gauge would make a shandy key chain accessory, or gentleman match shooter's pocket watch fob. Carry it to your next gun show. You'll be surprised how handy it is, and how often bore size is in doubt.