Ball Gauge, Precision Billetballgauge-BLK
Precision machined aluminum and deep penetration anodizing make this gauge a "must have" tool in your gear bag.
To measure a paintball: Place the ball on one of the bore sizings holes. When you find the hole that the ball just squeaks through without any assistance, that is the size of the round. Test 5-6 different paintballs to average the size.