Page "Fighter aircraft" Paragraph 79

Armament also advanced during the war.

The rifle-caliber machine guns that were common on prewar fighters could not easily down the more rugged warplanes of the era.

Air forces began to replace or supplemented them with cannons, which fired explosive shells that could blast a hole in an enemy aircraft — rather than relying on kinetic energy from a solid bullet striking a fuel line, control cable, pilot, etc.

Cannons could bring down even heavy bombers with just a few hits, but their slower rate of fire made it difficult to hit fast moving fighters in a dogfight.

Eventually, most fighters mounted cannons, sometimes in combination with machine guns.