I think that it needs to be noted that prior to and during WWII arial gunnery was taught by having an aircraft tow a target sleeve, like a big wind sock. To fire from directly behind was not allowed because of the likleyhood of shooting the tug aircraft down and scoring the shots. Scoring was done by giving each aircraft bullets with a different color dye on them and counting the number of marks of a given color. They normaly started their runs from an angle of about 30deg, judging from the accounts I've read, so deflection shooting was more or less taught.
This changed with the P-63 and "Frangable Bullets" (they shatter on contact with any resistance) which had a light bulb that flashed whenever the aircraft was hit. I don't know how many there were or if it was an experimental project or how widespread it was if it was used.