The AH-64D Apache gun

[PoorOldSpike]

You can either play as the pilot or gunner. Let's be pilot first-

Your gunner opens up as soon as he sees the enemy-

But if he's slow in spotting them you can kickstart him into action by panning around (in freeview '*' numpad) til an enemy marker appears, then click left mouse to order him to fire-

He might also go on a killing spree and switch to other targets-

 

[PoorOldSpike]

And now let's be the gunner-

The chin gun will swivel to point at whatever the crosshairs are placed on, even sideways like this, press left mb to fire..

Hit 'N' for DTV (Daylight TV) view-

Hit '+' (numpad) twice for full zoom, the gunfire will follow the crosshairs-

'N' will bring up Night vision-

And 'N' again will bring up Infrared-

A line of infantry is hosed down-

The right mb sometimes brings up targetting aid markers-

There's yet another way to play with the Apache weapons; do 'Player as pilot' then select 'Manual fire' and hit the '2' key to get a list of targets to order the gunner to fire at, but remember whatever method you use, AA2 is NOT an ultra-realistic true helicopter simulation and was never meant to be.
Personally I play AA2 as an INFANTRY game and regard helos and planes simply as useful tools that can be called in to clobber the enemy, i've got no real wish to actually board them.
I've got modern jet combat flight sims such as 'Wings Over Europe' for when I want to do that.
Incidentally I think there's currently a gap in the market for an ultra-real chopper PC sim such as the old 'Longbow 2'.