Understanding Less-Lethal Projectile Tolerances: Bore Sizing (.684/.685) & Mass Density

Projectile fit is not determined by caliber labeling alone. Two launchers marked .68 caliber can have different internal barrel dimensions, surface finishes, wear patterns, and retention systems. For that reason, AR Tactical uses a barrel-first approach to projectile selection.

Nominal Size vs. Actual Fit

A .684 round is nominally one thousandth of an inch smaller than a .685 round. That difference is small, but it can materially affect retention, blow-by, friction, velocity consistency, and feeding. Manufacturing tolerances mean the stamped or advertised bore size should be treated as a starting point, not a guarantee.

How to Perform a Basic Fit Check

With the launcher unloaded and depressurized, verify that the projectile is retained without excessive force and can pass through the bore without binding. Never force an oversized projectile into a barrel. Test a small sample before loading a full magazine because molded projectiles can vary slightly from piece to piece.

Mass Density and Energy

Mass density describes how much mass is concentrated within a given volume. Two projectiles of the same diameter may have very different weights because of their materials and internal construction. Muzzle energy depends on both mass and velocity, so a heavier round does not automatically produce a higher energy reading. Chronograph testing is the correct way to confirm actual performance.

Practical Selection Rule

Choose the projectile that feeds reliably, remains properly retained, does not bind, and produces consistent chronograph results in the exact barrel and launcher configuration being used. Platform names alone are not enough to establish fitment.

Safety First, Force Second: verify barrel fit, launcher condition, target system, and local law before use.