Sustained firing of a machine gun heats up the barrel remarkably quickly. The danger of this is that if the barrel becomes too hot, rounds can be detonated by heat alone, before they are fully seated in the chamber which is dangerous to the firer. On heavy machine guns this problem can be mitigated by having a water jacket for cooling, or vanes to pull heat away from the barrel. The barrel can also be of a heavier gauge which takes longer to heat up. Light machine guns are harder to cool as all these methods add weight to the gun and the whole aim of an LMG is to be as light and portable as possible. The Bren gun got around this problem by issuing a spare barrel for each weapon, with the weapon set up to allow them to be changed quickly. After firing ten magazines in rapid fire the barrel should be changed, as outlined in the manual:

No. 1 will, when the tenth magazine is empty, pull back the cocking handle, remove the magazine and say “Barrel,” at the same time closing the magazine opening cover.

No. 2 changes the barrel and, after No. 1 has opened the magazine opening cover, places a magazine on the gun. No. 1 continues firing

 Until fairly recently getting hold of deactivated barrels for Bren guns was very problematic but recently the rules in the UK have changed to make it much easier for deactivated barrels to be offered on the collector’s market and this is an example of a Bren Mk II barrel that matches my Bren Gun:

The barrels replicate a number of features on each one, so we have the front sight on its offset post:

And the gas plug that allows the rate of gas flow to be altered depending on the ammunition, fouling etc:

A handle is fitted to allow the barrels to be grasped when hot for quick changes to be executed:

Originally the barrel would have had a series of interrupted lugs on the end of it to allow it to be secured into the trunnion of the Bren gun, these have been ground flush as part of the deactivation process:

Having a real barrel I the spare parts bag, as opposed to a resin or wooden replica, is a step well worth making as the barrel is far more robust than any replica and has the correct weight which all of the replicas lacked. Very rarely do updates in deactivation laws benefit the collector, but in the case of spare barrels the ability to purchase legal examples is something that has been a boon.