Are you getting the most out of your long range precision shooting rifle? In this part of our series, I’ll start talking about long range rifles: how to select them, and how to improve their performance. Let’s start by discussing inherent accuracy and the main factor that determines it: barrel vibration.

Inherent Accuracy

The inherent, or intrinsic, accuracy of a rifle is its ability to shoot tight groups.  It is gauged by measuring the average group size at a given distance, usually 100yd or 100m, and is measured in MOA. It is measured as an angle because, in theory, if a rifle is capable of shooting 1MOA groups at 100yd, then it is able to maintain that angular spread at all distances. This is only the theory though. In practice, the spread widens as the distance lengthens. Occasionally, the grouping spread may be wider at shorter distances, due to dynamic stability issues, but that’s an exception to the rule of thumb. In general, measuring the group spread at 100yd is a good way to predict the accuracy of a rifle.

But what makes a rifle accurate?

Ammo quality and consistency have a lot to do with the level of accuracy you can achieve, but I will talk about it in a future post. For now, let’s just concentrate on the rifle.

pressure peakThe first thing you need to know is that when you shoot, the rifle is not static. The rifle begins to react to firing the ammunition as soon as the powder begins to burn, as it’s ignited by the primer. The highest level of chamber pressure is reached in the first instants after the powder ignition. The graph to the left shows the acceleration, in G, of the rifle under the effect of that peak of pressure. On the X-axis there is the travel of the bullet, in inches, while on the Y-axis there is the amount of acceleration. As you can see, the highest acceleration occurs before the bullet has even traveled two inches.  This acceleration, and the force of the bullet traveling through the bore, both result in barrel vibration.

Barrel vibration affects the overall accuracy of the weapon system because it changes the way the bullet leaves the muzzle. This produces shifting in POI (point of impact) from shot to shot and, and consequentially, causes grouping spread on the target.

Barrel Vibration Theories

There are two different theories about the barrel vibration, both with observable confirmations in real world.

The first theory I call the theory of the harmonics, assumes the barrel vibration causes a weaving movement of the barrel, with the muzzle weaving and the line of departure shifting from pointing upward to downward at a constant ratio, or frequency. The barrel acts like a plucked guitar string as a chord is strung or, since it is tied only at one end (to the receiver), like a ruler positioned at the edge of a table.