Anti-aircraft warfare
AA gunnery was a difficult business. The problem was of successfully aiming a shell to burst close to its target's future position, with various factors affecting the shells' predicted trajectory. This was called deflection gun-laying, 'off-set' angles for range and elevation were set on the gunsight and updated as their target moved. In this method when the sights were on the target, the barrel was pointed at the target's future position. Range and height of the target determined fuse length. The difficulties increased as aircraft performance improved.

Q: What was the problem with AA gunnery?
successfully aiming a shell to burst close to its target's future position