Concentricity is the measurement of how well the centerline of the bullet aligns with the centerline of the cartridge case. In a perfect world, these two axes would be identical. In the real world, manufacturing tolerances and sizing operations introduce "runout"—the degree to which the bullet is canted or offset from the centerline.

The Impact of Runout

If a bullet is seated crookedly in the case, it will enter the rifling at an angle. This "canted" entry causes the bullet to become slightly deformed as it is swaged into the grooves. This deformation unbalances the bullet, causing it to "wobble" during flight (precession), which significantly opens up group sizes at longer distances.

Identifying the Geometric Source

To solve a runout problem, you must perform a geometric audit of your process. Use a concentricity gauge to measure the runout at each stage:

1. Fired Brass: If fired brass from your rifle shows significant runout, your chamber or bolt face may be out of square. This is a job for a qualified gunsmith.

2. After Sizing: If the runout increases after sizing, the issue is your die or press. Often, an off-center expander ball in a standard die is the culprit, "pulling" the neck out of alignment as the case exits the die.

3. After Seating: If the runout only appears after seating the bullet, the problem lies in the seating die or the relationship between the seating stem and the bullet tip.

When group sizes open up unexpectedly, follow a systematic checklist rather than making random adjustments to your load.

Mechanical Integrity (The "Low-Hanging Fruit")

Before questioning your handloads, verify the integrity of the rifle itself. A loose screw in a scope base or ring is the single most common cause of sudden accuracy loss, and it is the easiest to fix: check all mounting hardware with a quality torque wrench, applying the manufacturer’s recommended values (typically 15–20 in-lbs for rings and 30–45 in-lbs for bases). Similarly, ensure the action screws that secure the barreled action into the stock are properly torqued. Variations in action screw torque change the bedding stress on the receiver and barrel, which in turn alters the barrel’s vibration nodes and shifts the point of impact.

Finally, inspect the muzzle crown for nicks, dings, or carbon build-up. Even a tiny defect at the exit point of the barrel can unevenly vent the propellant gases that are still accelerating the bullet, “kicking” it sideways as it leaves the muzzle. A damaged crown is easily identified with a magnifying lens and can often be re-cut by a competent gunsmith without re-barreling.

Analyzing the Group Shape

The shape of your groups on the target provides diagnostic clues to the underlying problem—but only once enough shots have been fired to make the shape mean anything. Group shape is dominated by sampling noise at the sample sizes commonly used at the bench, and the reader who skips this paragraph will diagnose noise.

Consider a rifle with no directional error whatsoever, whose dispersion is perfectly circular. Its groups still look strung. At three shots, 80 % of them show an elongation of 2:1 or more, and only one in five looks round; because the long axis falls in a uniformly random direction, 20 % read as “vertical stringing” and another 20 % as “horizontal”. At five shots it is still 13 % each. The pattern only becomes evidence at around twenty shots, where fewer than 2 % of groups from a circular process reach 2:1. This is the same small-sample trap that Appendix [app:statistics] treats for velocity: three or five shots estimate a mean tolerably and a dispersion not at all.

Read the table below, therefore, as a list of hypotheses to test—by firing more, or by changing one variable and firing again—never as a diagnosis to be made from a single small group.

Vertical stringing is the most common pattern and typically indicates a velocity variance. Check your powder charge consistency, neck tension uniformity, and primer seating depth. Vertical stringing can also result from inconsistent barrel temperature: as the barrel heats up during a string, the point of impact may “walk” vertically. Allow sufficient cooling time between shots during load development to eliminate this variable.

Horizontal stringing is most commonly caused by wind or by inconsistent rifle positioning on the bench—particularly side-loading the rear bag. If horizontal stringing persists even in calm conditions, suspect a loose scope mount, an improperly torqued action, or a barrel that is making contact with the stock at the forend. A simple diagnostic test is to slide a strip of paper between the barrel and the stock channel; it should pass freely along the entire length without binding.

Diagonal stringing frequently indicates a bedding issue or a barrel that contacts the stock intermittently as it heats up and expands. Diagonal dispersion can also be caused by a scope with an internal tracking problem, where the erector tube shifts slightly under recoil.

A circular “shotgun” pattern—uniformly large in all directions, without a clear directional trend—typically indicates a fundamental concentricity problem in the ammunition. Check bullet runout with a concentricity gauge. If the ammunition measures well, inspect the muzzle crown for damage.

Finally, if your shots consistently cluster into two distinct groups (a “dual point of impact”), the most likely cause is a scope whose erector tube is shifting under recoil and returning inconsistently. This is more common in lower-quality optics and can be confirmed by testing with a known-good scope. If a known-good scope reproduces the same two groups, look at the bedding: a pinch point in the epoxy—common where a factory has bedded only the recoil lug—gives the action two resting positions instead of one, with the same signature.

Reflection: As Sinclair and other legends of the benchrest world often said, "the target never lies." If your groups are opening up, the rifle is telling you that something in the system has changed. Your job is to be the detective and find the geometric anomaly that is disrupting the system.