airsuperiority
Captain
Attack helos typically want higher cal cannons for range and punch. The bigger and heavier the round, the longer the effective range and harder it hits, which works equally well no matter if you are targeting heavy armor or light infantry, not that the cannon was ever designed to take out single infantrymen.
The cannon on an attack helo is really a surgical weapon and tend to be pretty accurate, which is also why they typically only carry hundreds of rounds, not thousands, and why attack helo pilots always fire the cannon in short, controlled bursts instead of just hosing all over the place. If an attack helo wants to do area suppression to deal with a large number of scattered light targets, it won't waste time and rounds plinking away with it's cannon, it will let rip with a salvo of rockets, which can easily cover an area the size of several football pitches, that's why they carry those.
Another reason you would not really want to go below 20mm is because it's better to overkill than be caught short. If you only equip your attack helo with 14.5mm, and it runs into a situation where 14.5mm just doesn't cut it, its screwed. Whereas 20mm would work just as well against targets that only need 14.5mm.
Another lovely post. What you've said got me thinking, and it's true. Footages of attack helos usually show them pop a burst at the target and it always land where they want them. And let's not forget that heavy attack helos such as the Apache are known as tank-killers for a reason too. if anyone's going to equip anything lighter, it's already for much lesser target, and they can use the .50cal or even the 7.62 from the windows. and usually you find these on black hawks and other transports