What is a buddy bottle or external tank tether?

A buddy bottle is a removable air bottle that screws onto the gun in place of an integral tube, often holding 300 to 500 cc, which lets you swap in a fresh full bottle in seconds.

Tethering means leaving a fill hose connected from a larger tank to the gun while you shoot, so the gun is constantly topped off. Tethering is great for benchrest and tuning but limits mobility. Both approaches extend shooting sessions far beyond a single onboard fill.