In this image, the detachment is under strength, and has two positions vacant. Russell Chisnall is the No. 3 who actually fires the gun.
Behind him is Dave Williams, the No. 4, loading, and to the right, Cameron Kinnear, the No. 2, who is operating the breech.
In the Ubique Group all members are trained to perform all the positions in the detachment, and take turns in those positions.