With the drum fitted there were no sights so aiming, such as it was, was with tracer rounds, hosepipe fashion. The use of the Bren for AA fire ceased in 1940 following reports that there was somethjing quite uncomfortable trying to hose down aircraft that were up there just looking for trouble - and you were giving them some! The down side was that could fight back - and inevitably did with 20mm cannons at 2 miles away!
The Bren was soon relegated to doing what it did best. Indeed, it excelled at..... A section machine gun.
There's a little bit of irony in this fact. The little stud at the front left side of the butt slide, that retained the gun into the tripod AA mount was retained right up until the very last Mk3 gun butt slides were produced/cast as spare parts in 1976. Somebody forgot to tell them to delete this redundant feature from production.
Not a lot of people know that..........