Initially it was a roughly rectangular pattern (I think about 4' x 4') at 400 yds to give 'the average pilot' a chance of hitting the target. It was later reduced to point harmonisation at 250yds.
I can't locate any official documents just now but this may help:
http://books.google.co.uk/books?id=v...rgence&f=false
The ammo loadout is interesting but I know that changed quickly to include DeWilde incendiary when it became available. TBH my loadout does not include Ball, I use 4xAP, 2xTracer/Incendiary and 2xDeWilde and I imagine that may have been frowned on, possibly even due to cost.