What is the concept of Optimal Barrel Time (OBT)?

The concept

Optimal Barrel Time (OBT) is a concept in internal ballistics that describes the ideal moment for a projectile to exit a rifle barrel so that accuracy is maximized. When a rifle is fired, rapidly expanding gases accelerate the bullet down the bore, while the barrel itself begins to vibrate like a tuning fork. These vibrations, often called barrel harmonics, cause the muzzle to move in small but measurable patterns as the bullet travels toward the exit. Because the bullet’s direction is set at the instant it leaves the muzzle, any muzzle movement at that moment can shift the point of impact.

Nodes & Sweet Spots

OBT suggests that there are specific “sweet spots” in time—measured in milliseconds—when the bullet should exit the barrel to coincide with a stable portion of the barrel’s vibration cycle. Typically, this stable portion is near a vibration node, where muzzle movement is momentarily minimal or changes direction. If a bullet exit occurs during this calmer phase, small variations in velocity or pressure will have less effect on shot-to-shot consistency, improving group size.

Application

Shooters and reloaders apply OBT by tuning ammunition parameters to match the barrel’s harmonic behavior. Adjusting powder charge, burn rate, bullet weight, seating depth, and even primer choice can change muzzle velocity and, therefore, the time it takes for the bullet to travel the barrel length. By finding a load that aligns exit timing with an optimal harmonic point, shooters can often achieve tighter groups without changing the rifle itself.

While OBT does not eliminate all accuracy variables—such as wind, shooter technique, or inconsistent components—it provides a practical framework for systematic load development. In essence, OBT links the physics of barrel vibration with ammunition tuning, offering a science-based explanation for why certain loads perform exceptionally well in a given rifle.

Relationship of OBT Nodes and Barrel Length

A lot of the research work and the theory was developed by the Long Family: http://www.the-long-family.com/OBT_paper.htm. They also developed one of the most commonly used tables of OBT times vs. Barrel length ( http://www.the-long-family.com/OBT%20Table.pdf ):

At QuickLOAd PRO, we use the following formula to determine the nodes:

OBT(node) = (slope × L) + intercept

Where the coefficients for each node are:

Questions and answers:

Q: Does OBT always work as described above?

A: The model assumes some simplification. Hence, it is possible that the nodes do not match real-world results, particularly if the gun has elements that impact the shock waves, i.e., (large/heavy) silencers. Based on our experience, the concept also works best for barrel lengths between 16 and 26 Inch.