Some Observations on Target Range Grouping; Bullseye Shooting Accuracy, Shot Strings & Groups, Minute of Angle & More

Shooting Range Industries provide custom made indoor shooting ranges, and therefore we have an interest in targets. Shooting range rules usually dictate that paper is used so let’s start by looking at the humble bullseye target can be utilized to evaluate our accuracy, precision and ammo performance.

Bullseye Target Shooting Accuracy

First, precision and accuracy. These terms are generally used interchangeably but actually mean two different things. Accuracy is the ability to hit the target, using the NRA B6 or B8 an accurate shot is to the X-Ring, which is 1.695 inches in diameter. Regardless of the group size, if the hits are in or around the bullseye or X-Ring, you are shooting accurately. Precision is repeatability. When you shoot a three string group that clover leafs, meaning the group is overlapping, you are shooting with precision. If the group is clustered out at about the 8 or 7 rings, up to 5.5 inches from the bullseye, you are shooting with precision, but not with accuracy. To shoot with both accuracy and precision you want a tight grouping centered on the X-Ring.

Shot Strings

Statistically speaking the larger samples provides a higher degree of confidence. More is usually better, but economically a 100 round test may not be feasible. So how many shots do you want in a shot string? What is our goal? If we are testing a hunting load, we want to duplicate as close as possible realistic field conditions. If you are a hunter, in all reality you will probably not shot more than a three shot string. Starting with a clean barrel you fire a three shot group, and you may want to shoot 3 – 5 groups, cleaning and allowing a 10 minute cool down for a meaningful insight into your ammunition’s performance.

How to Measure Shot Groups

How do you measure your group? There a various thoughts on this, but center to center measurements to the 1/16th of an inch with a tape measure or perhaps to the 32nd of an inch with a caliper will give you sufficient information to evaluate the groups. Simple, accurate enough and to the point, and will work with any caliber.
Testing for handgun competition you may want consider 5 or 10 round groups as 10 or multiples of 10 is the usual number shot in each stage, depending on the nature of the competition.

Calculating Minute of Angle

Range in testing is an important consideration. A rifle bullet ‘wobbles’ to some on its exit from the muzzle, this wobble dampens out with range. A gun may be more accurate at 100 yards than at 50 yards. The wobble is technically the combination pitch (up and down) and yaw (side to side) motion from the center of the spinning bullets path. One of the more useful terms to the shoot is MOA or Minute of Angle. A minute of angle is 1/60th of a degree and an arc and is about 1” at a 100 yards, 2” at yards, etc.. If a gun shots to 1 minute of angle, all shots will cluster in a diameter no greater than a 1 inch. This measures repeatability and precision of the rifle and ammunition combination.

Horizontal & Vertical Bullet Stringing

Shot strings can tell us a lot. If we do our part, putting the sights on the cross, firing from a bench rest we will see patterns develop. If the string is wider horizontally and minimized in the vertical, this indicates that the wind is affecting the bullet. It is good to know how your chosen bullet reacts to the wind, so you can compensate for this in the field. Vertical strings can indicate variations in the powder load. The closer the minimum and maximum in the string’s load are to the average, the lower and tighter the vertical string. If you sight in your rifle at 100 yards it provides a very good base for future range or wind adjustment. Scope adjustments are usually in increments of a 1/4th minute of angle to a finer 1/8th MOA.