Bore evacuator

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A bore evacuator is a device on the gun barrel of an armoured fighting vehicle which helps prevent poisonous propellant gases from venting back into the vehicle's fighting compartment when the gun breech is opened to load another shell. Bore evacuators are most often used on the large-calibre guns of tanks and self-propelled guns.

The evacuator is a reservoir that holds the super-heated, high-pressure propellant gases produced by the firing of a shell, then releases them as the shell exits the barrel. When not in use, the atmosphere inside the barrel remains the same as the surrounding environment, thusly it is, or nearly, the same inside the evacuator.

As the shell passes through, an opening into the bore takes in the gases, containing them until the shell has exited, then releases them back into the barrel. The openings are angled toward the muzzle, so the stream of still fairly high pressure gas drags both combustion gas in the barrel and fresh air from the open breech toward the muzzle. This reduces the chances of these explosive propellents to flow back into the cabin and cause combustion as they mingle with oxygen, though this can still happen if the evacuator is poorly designed. It is becoming a common feature of most modern tanks.