Machineguns fire mode.

[Targan]

Originally Posted by Mahatatain

According to my father (who I must stress is not a military expert of any kind) to make the Bren gun spray the rounds in a burst and not just put them all through the same hole the designers had to loosen the fixing where the bipod attaches to the gun to make it move around a little, essentially making the Bren gun rock slightly on its bipod.

I've heard a similar anecdote. I think your dad might've been on the money, or close to it.