What is The Lump in a Tanks Barrel?

Tanks are complicated pieces of engineering, they are designed to mount offensive firepower on a mobile platform while providing defensive protection for the crew within.

In order to achieve those requirements, a tank is usually an assembly of complex technology, systems and hardware. Upon visual inspection, it is not always obvious how some of these components operate, or what their function is.

A commonly misunderstood feature of a tank is the bulge in the barrel seen on most modern tanks with large calibre guns. This bulge is a bore evacuator, also known as a fume extractor.

A bore evacuator can be seen near the muzzle on this Type 59’s 100 mm gun.

Why its Needed

When a large calibre gun is fired, the combustion of the propellant releases large amounts of gas and smoke. In a tank, when the breech is opened to reload, this gas enters and quickly fills the confines of the crew compartment.

This can cause significant issues. The fumes themselves are toxic and can cause crew members to become nauseated, while the thick smoke can significantly reduce vision. Enough gas can reduce their capacity to operate effectively.

The gasses can also contain unburnt propellant that is still explosive. Under the right conditions this can detonate within the turret, potentially injuring or killing those inside, this is known as a flareback. 

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Around the end of World War Two, tank designers began using bore evacuators to combat this problem.

How They Work 

A bore evacuator is a very simple design without any moving parts, instead using pressure differentials to suck out the smoke. It is a hollow chamber sealed to the barrel of the gun. A ring of holes are drilled through the barrel into the chamber.

The simple construction of the evacuator can be seen on this cutaway of a 105 mm L7 gun.

When a round is fired, the combusted propellant produces a massive amount of pressure behind the shell, forcing it through the barrel. Once the rear of the shell passes the bore evacuator holes, the high pressure gas quickly fills the chamber. 

High pressure gas rushing into the evacuator

As the shell approaches the end of the tanks barrel, the pressure in the barrel starts reducing, until almost reaching atmospheric pressure when it leaves. However, the bore evacuator chamber is still under high pressure. 

Barrel pressure reduces

The gasses contained inside the chamber rush back into the barrel through the holes. The holes are angled towards the muzzle, causing the gasses to flow that way. This rush of gas out the barrel pulls any lingering smoke with it. 

As this is happening, the breech is opened, allowing fresh air in to help push the unwanted fumes out.

High pressure air shooting towards muzzle, while fresh air rushes in

It must be understood that this entire sequence happens within a fraction of a second.

As the breech needs to opened at a specific point; when the gasses leaving are travelling the fastest, this system usually requires a semi automatic breech to work best. This system almost eliminates smoke in the fight compartment, and its adoption has significantly improved crew conditions.


Some vehicles like the M1A1 or M1A2 Abrams have whats called an eccentric bore evacuator. An eccentric evacuator is offset to sit high on the gun instead of being wrapped equally around it. This allows the gun to depress lower.

An eccentric bore evacuator as seen here on an M1A2 Abrams.

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While this smart use of pressure differentials can’t be observed while in action, its results can be, watch for a puff of smoke out the end of the tanks barrel, just after the initial fireball.