you only have the two buttons. One for firing the primary weapons, and one for the secondary. Secondary weapons are everything OTHER than the main guns. use the "Z" key to zoom, in on the dashboard. You will find two white display that show how many rounds you have. While watching the "Sec" window scroll thru the avalible weapon choices by hit the "backspace" key.
In the case of a fully loaded P38, the primary always showes the 50 cal rounds, the secondary will scroll thru cannon rounds, bombs, drop tank, rockets. Which ever weapon is being displayed will be what you fire when you hit the "secondary weapons"button.
