Ammo Differences

Anyone got the lowdown on Ammo differences for Cannons / Machine Guns?

I've unlocked Air ammo, AP, and Tracers for various planes but there's also Universal and "default".

I have no idea what's most effective in which situation, besides trial and error.

It seems to me that Air ammo seems better (faster ttk) on planes and AP seems better at ground vehicles, but is there a guide anywhere, so I can learn more about this stuff I am unlocking?