Mmmmmm. There are two major things that make a Bren stop Insufficient energy available to unlock, extract eject and cock. The second is insufficient energy to feed, load, lock and fire. Cause of 1 is insufficient gas and cause of 2 is insufficient stored energy in the return spring. From that, the answer is that we need to know at what point it actually stops. Any further thoughts Son?

One point that isn't well known regarding Brens is the friction imparted on the breech block by the action of the locking ramp on the piston extension. The ramp forces the locking cam at the back of the bolt to bear hard on the underside of the body. This is deliberate friction that gives the Bren its slow (?...., well, relatively slow....) rate of fire. It eats up a lot of the stored energy held within the retutrn spring. So on the face of it, the spring can seem perfect but is too weak to overcome this additional friction. That's enough of the physics lesson!