Bore evacuator

By creating a pressure differential in the barrel after the shell leaves, the bore evacuator causes most of the propellant gases and combustion residues to exit via the muzzle.

When the gun is fired, high pressure gas generated by the burning propellant pushes the projectile forward.

[2] For best results, the breech must be opened at the proper time, just as the forward momentum of the gases reaches its maximum, the peak flow.

This means that bore extractors are normally used only on guns with semi-automatic or fully automatic actions, where the breech is opened and the shell ejected as part of the recoil process.

The French Leclerc tank is a counterexample; it instead uses overpressure from an internal compartment to force gases outward.