High explosive 20mm Shells and convergence

[Winger]

Hello folks.

Could anyone please tell me if its important that the target i shoot at with my 20mm rounds is in perfect range. In other words: Do the shells also explode on impact leaving it no diffrence if i am at 10m range and score a hit or at just the range at wich the shells explode themselves (like 200m?)?
If it is important to be at the right distance then: Is there a way to somehow "adjust" my bullets to explode at a certain distance?

Thanks!

winger