You're right ZGB. The troubles is that as good and thoprough and detailed as trials are, they can never replicate conditions in the field and later, at war. When you go to war, you go with what you've got. It was only after the Brens were used under dire conditions when due to constant use and abuse, they couldn't be cleaned that the gas fouling and carboning up became apparent.

Don't forget that there's TWO gas ports in the barrel (?) The gas port proper and the what is called the 'nozzle' gas port that directs the gas from the gas regulator down to the face of the piston. Realistically, you could have as much gas as you like venting from the barrel to cycle the action, but as many have learned from their L1A1 rifles, a minute split in the gas cylinder - or in the case of the Bren, the merest loss of gas from a leaking gas from the bipod sleeve/gas cylinder taper spells doom. And so it goes on........, getting worse.

Open a bigger gas regulator hole - , more gas. More gas, more carbon. More carbon, more fouling until you end up with nothing more than a heavy single shot rifle. And not even an accurate one either!