"Tuning the bolt and valve system
In mechanical and poppet-based electropneumatic markers, the valve is usually designed to accommodate a specific operating pressure. Low pressure valves provide quieter operation and increased gas efficiency when tuned properly. However, excessively low pressure can decrease gas efficiency as dramatically as excessively high pressure.
Additionally, the valve must be set to release enough air to fire the paintball. If the valve is not tuned properly, insufficient air to fire the paintball may reach the bolt. This phenomenon, known as “shoot-down,” causes fired paintballs to gradually lose range, and can also occur at high rates of fire. Some markers have integral or external chambers, called low-pressure chambers, which hold a large volume of gas behind the valve to prevent shoot-down.
Tuning can also prevent air blowing up the feed tube upon firing, which disrupts the feeding of paintballs into the marker."