How-To Advice: Measuring Bullet Groups Accurately
Many shooters dont know the best way to size their accuracy performance consistently and correctly. Heres how to do it.
The Neil Jones target-measuring tool allows
This is not a matter of earth-shaking significance. The national deficit wont go up or down because you consistently measure groups a hundredth of an inch smaller than they really are. But it does hamper communication and learning about shooting when you and I cant be sure what the other person means when stating a group size. We can only judge what really works to improve accuracy if we have a consistent standard to go by.
Heres how to determine that standard.
A normal group measurement is the distance between the centers of the two most widely separated bullet holes. This is a good basis of comparison because it is not influenced by the caliber of the bullets, but there is a problem with it: The center of a hole is not a precisely defined point from which we can make simple measurements. To get around this problem, many shooters measure between the outside edges of the two widest holes, then subtract a bullet diameter. Yet, while this technique is sound in principle, it does not give accurate group measurements for two basic reasons.
First, you cant be sure of the exact outside edge of the mark left by a bullet on paper. Second, paper stretches when a bullet passes through it, so the mark left by the bullet doesnt have the bullets precise diameter. These two factors combine to make the figure produced by outside-edge measurement and center-diameter subtraction smaller than actual group size.
A BETTER SOLUTION
But there is a way to obtain accurate group measurements. Its an elegantly simple method devised by benchrest competitors many years ago: Visually centering the bullet holes in a circular reticle.
If you scribe a circle of approximately the same diameter as your bullets on a piece of transparent material, then lay that material on a target, youll be able to center a bullet hole in the circle within a few thousandths of an inch. This is important because you have also located the holes center to the same degree of precision. All you need now is a way to measure how far the circle travels when you move it to a hole on the other side of the group.
The Neil Jones Custom Products target-measuring fixture shown in the accompanying photographs is based on this simple principle.
The Jones fixture consists of two main assemblies. The first is a block that attaches to the fixed jaw of a dial or digital caliper. It contains a spring-loaded plunger with a sharp point. The second part attaches to the calipers movable jaw assembly, and holds a clear plastic reticle plate that is scribed with several circles of different standard bullet diameters along a visible, straight line.
The fixture is simple to use. Place your target on a flat surface. Turn it so an imaginary line connecting the two most-widely spaced bullet holes is horizontal. Make sure the caliper jaws are fully closed, then center the appropriate circle around your leftmost bullet hole while aligning the reticle plate so the scribed line passes through the approximate center of the rightmost hole. Without disturbing the calipers position, push down and hold the plunger so its sharp point anchors caliper and target to the flat surface. Open and adjust the caliper until the same circle is centered around the rightmost hole. Then, all you need to do is read the group size directly from the caliper.
Simple as this measuring process is, there are still two potential problems. You will get slightly different readings every time you measure the same group, due to changes in light and the way your eyes perceive the reticle and bullet holes. So dont just make one measurement. Make two or three, starting from scratch each time, and use the average. Also, with really good groups, you wont have separate bullet holes to measure from. Youll have a single irregular cloverleaf. In this situation, all you can do is align the reticle circle with the partial bullet holes at the outside of the group as best you can. With a little care, this should still be enough for precise results. Just maintain the same even contact or spacing between circle and partial hole. Dont merely bring the reticle into contact at a single point.
I get more consistent results if I use a reticle circle thats a little larger than the bullets Im shooting. I can center the circle more easily by maintaining an even gap around the hole than I can with the circle covering the rim of the hole. You may prefer using a circle of exact bullet diameter. Regardless of your choice, remember to position the circle so its centered on the hole rather than contacting it on one side.
Any magnifier that leaves your hands free to operate the caliper will make precise measurement easier. Neil Jones offers an optional lens that attaches to the target measuring fixture shown here. It is quite handy to have around.
The three best ways to get accurate results are practice, practice, and more practice. With experience, repeated measurements of the same group should vary no more than two or three thousandths of an inch. When you reach that point, you will be measuring groups about as well as the job can be done.
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-By Dan Hackett