Bore evacuator

A bore evacuator.

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 large-calibre tank guns and self-propelled guns. Without bore evacuators, hot gases and other combustion residue leaks into the tank's interior, depleting oxygen levels and filling it with a foul odour that can easily induce nausea and distract the crewmen from their tasks. (Perrett 1987:29)

Foreground, a close-up of the bore evacuator on an M1 Abrams, while another fires in the background

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; thus it is the same, or nearly so, inside the evacuator.

The barrels of older tanks, such as the IS-3 in the foreground, lacked the distinctive bump of a bore evacuator. The tanks in the background have them.

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 propellants flowing backwards into the turret, causing combustion as they mingle with oxygen, though this can still happen if the evacuator is poorly designed, poorly maintained, or damaged. Bore evacuators are becoming a common feature of most modern tanks.

One of the earliest guns to use a bore evacuator was the Royal Ordnance L7 used on the Centurion tank.

References

• Perrett, Bryan (1987). Soviet Armour Since 1945. London: Blandford Press. ISBN 0-7137-1735-1.