Apparently, officers could and did give their ground crews orders to adjust the aim of the guns to suit their preferences and experiences, usually abandoning the official 'spread' for convergence at 100 yards. Sergeant Pilots didn't have the same authority, so had to work with the standard settings. Consequently, Sergeant Pilots didn't so so well at getting high scores.
The concept of the cannon-armed fighter was proposed before WWII, and the HS404 autocannon was fitted to the Beaufighter in 1940. The Battle of Britain showed that the 303 round was not adequate for the task, and so the move to equip the single-engined fighter force with cannon ramped up.
Unfortunately, there was an issue with the HS404 that meant that in combat the guns could misfire. These failed guns were taken out of service and passed to the Royal Small Arms Factory for investigation. This job fell to my father, who was given a building full of these guns, and he spent some time finding a cause. I recall him telling me what he'd discovered, but it didn't appear to be the reason later put forward by officialdom. The official view was that the rounds were being too lightly struck by the (fairly light) bolt, the solution being to machine
1/16th or so from the face of the receiver, so that the bolt would be travelling faster and make a misfire less likely.I understand that something like 100,000,000 rounds of 20mm ammunition had been made in advance, and this was put at risk by the unreliability of the guns.
The Americans also produced the HS404, and they were still debating the misfire issue at the end of the war, although they had gone some way to adopting the British solution to the problem.
Here's 3m46 of British ground attacks in 1944. Locomotives made attractive targets...