its a set of calipers with an attachment that has an anchor and caliber specific circles inscribed on it .set the circle of your caliber on the Bullet hole ,anchor and measure to the longest point using the same circle centred over the farthest point , very accurate and fast to use . Biggest problem with trying to use just calipers and math is determining the edge of the bullet hole correctly and measuring directly across to the furthest point,Tends to really skew results .
Regardless , you shot some good groups keep it going
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