Cold War, Modern Day

What is The Lump in a Tank’s Barrel?

Tanks have a tough job. They are designed to mount offensive firepower on a mobile platform while providing defensive protection for the crew within. And ideally, it wont break while doing it.

In order to achieve those requirements, a tank is usually an assembly of complex technologies, 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. That covers the “what it is”, but continue on to find out how these work and why they are useful.

T-72 barrel
A bore evacuator (highlighted in red) can be seen on this T-72’s 125 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.

At the very minimum this causes discomfort for the crew, making both breathing and seeing more difficult. This can result in reduced ability to operate the vehicle.

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

Around the end of the Second World War, tank designers began using fume extractors to combat this problem.

How it Works

A fume extractor is a very simple component without any moving parts. It instead uses pressure differentials to suck out the smoke.

They consist simply of a hollow chamber sealed to the barrel of the gun. A ring of holes are drilled through the barrel into the chamber.

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

When a round is fired, the propellant within its shell casing ignites, producing rapidly expanding gasses that put a huge amount of pressure behind the projectile. This high pressure gas pushes the projectile through the barrel.

But, as we mentioned, we really don’t want these gasses entering the crew compartment. This is where the fume extractor comes in.

Once the rear of the shell passes the fume extractor holes, the high pressure gas quickly fills the chamber.

Fume extractor diagram 1.
High pressure gas behind the shell rushes into the extractor.

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

Fume Extractor diagram 2.
Barrel pressure reduces. Evacuator still under high pressure.

The high pressure gas contained inside the extractor chamber then rushes back into the barrel through the holes. The gas quickly flows towards the muzzle to reach the atmosphere.

This rush of gas out the barrel pulls any lingering smoke with it.

As this is happening, the breech is opened. Fresh air enters the barrel and helps to push the unwanted fumes out.

Fume extractor diagram 3.
High pressure air traveling towards the muzzle, while fresh air rushes through the open breech.

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. Fume extractors eliminate smoke in the fighting compartment almost entirely, and its adoption has significantly improved crew conditions.


Fume extractors come in all different shapes and sizes and can be found on any portion of the barrel.

Some vehicles like the M1A1 or M1A2 Abrams have what’s called an eccentric fume extractor.

Fume extractor on a M1A1 Abrams.
An eccentric bore evacuator as seen here on an M1A1 Abrams.

An eccentric extractor is offset to sit high on the gun instead of being wrapped equally around it. This reduces the amount a fume extractor hangs below the gun, allowing it to depress lower without striking the hull.

Another Article From Us: Zimmerit: Why do Tanks use Angled Armor?

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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.