That is the first time I ever saw the rapid fire with the middle finger on the trigger. Is that the way they trained? that is a cool design if planned that way.
Yes, this is common and Commonwealth Soldiers were trained to use the middle finger for rapid fire. During WW1, the story goes, that a German ground force were met with such rapid fire they assumed it was machine guns, it was in fact British soldiers using "mad minute" techniques with SMLE's.
Using the middle finger for rapid fire is as old as the hills in club rifle shooting in Australia ...