Historic Spitfire Mk1/1a loadout

[winny]

Early BoB - 3 guns loaded with ball, 2 with AP, 2 with Mk IV incendiary tracer (smoke tracer) and 1 with Mk VI (De Wilde) incendiary.

Later on (when De Wilde / Dixon were more common) 4 guns with ball, 2 with AP and 2 with incendiaries (Mk VI) with 4 of the last 25 rounds being tracer (Mk IV incendiary/tracer) to let the pilot know he was running out of ammunition
(although some pilots didn't like this as they thought it also let the enemy know too!)

Initially the guns were aligned to fire a pattern (not converged into one point) but this changed pretty quickly to convergence at 300 yards down to 200 yards, some pilots had them converged at 50 yards ( Flt Lt Pete Brothers being one of these)

Source : Fred Roberts Book "Duxford to Karachi - An RAF Armourer's War"
And "Flying Guns of WWII" by Anthony G williams

It was also dependant on rank, Sergeant Pilots were often told how their guns were set by the Armourers (same rank) whereas the Officer pilots could order the armourers to do what they wanted.
Lots of interesting stuff in there for anyone interested.