Barrel Bore

Barrel bore refers to the internal diameter of the smooth section of a paintball barrel that the ball travels through before exiting the muzzle. This diameter is usually expressed as a decimal figure in inches, and many modern barrels and paintballs fall somewhere in the high .670s to low .690s range. Exact values vary by manufacturer, paint batch, and environmental conditions.

The relationship between paint size and bore size has a direct influence on how efficiently a marker uses gas and how consistent velocities are from shot to shot. When the bore closely matches the actual paint diameter, the paintball forms a relatively uniform seal against the barrel walls. This helps air act on the ball more efficiently and can improve velocity stability across a string of shots.

Underboring occurs when the selected bore is slightly smaller than the average paint size. A modest underbore can improve sealing and reduce the amount of air that escapes around the ball, potentially increasing efficiency and tightening velocity spreads. However, if the fit becomes too tight or the paint is brittle or inconsistent, friction rises and the risk of barrel breaks increases.

Overboring occurs when the bore is slightly larger than the paint. This reduces direct contact and friction, which can be more forgiving when paint size varies due to humidity, storage, or manufacturing differences. The trade-off is that more air can slip around the ball, which may lower efficiency and make velocity more sensitive to changes in paint size or temperature. Some players prefer mild overboring for reliability with fluctuating paint conditions.

From a mechanical standpoint, bore geometry and consistency matter as well. Uniform diameter, roundness, and surface finish influence how smoothly the ball accelerates and how predictable its exit velocity will be, especially at higher rates of fire where small variations can accumulate. A consistent bore helps gas flow more evenly around the ball and reduces internal turbulence.

Barrel kits and insert systems give players the option to adjust bore size to match the paint available on a given day. Common approaches include simple fit tests, such as placing a paintball into an insert to see whether it rolls through under its own weight or requires a gentle blow to move. These quick checks help select a bore that balances sealing, friction, and reliability for current conditions.

Because paint size, temperature, and humidity all change over time, there is no single perfect bore size for every situation. Instead, players aim for a combination that supports efficient gas use, consistent velocities, and low break rates while fitting their marker’s characteristics and the quality of paint available.