I was on F4's for several years at Coningsby, Wildenrath, Stanley & MPA, as an armourer.
The SUU gun had six barrels, the primary barrel was harmonised, then barrels 3 & five were harmonised to the primary. This meant that the rounds fired from the primary barrel went where you aimed, but the rest went in a spread around the aiming point, a bit like a cross beween a rifle and a shot gun.
The Gun in The SUU Pod had a clearing cam and a firing cam, when you stopped firing, the live rounds went to the back of the gun and were ejected out the bottom as live rounds when you started firing again. That is why you did not count every round as fired in your calculations.
Some crews were natural gunners, others not so, this occasionaly led to us being requested to load a "Tartan" belt into the A/C of a good crew, ie the 50 rounds visible in the conveyor being the colour as stated in the 700, whilst the remaining rounds in the drum were the colour of the crew who were having trouble hitting the banner.
Sometimes the team collecting the dropped banner and taking it to Ops for scoring had to take out a tin of tipping paint and a broom handle...