As BaldEagl said, there is a vertical component.
It gets real confusing when you think what happens as a result of that when you bank your airplane, or even fire while inverted.
Firing while banked left will make your rounds hit WAY left, and LOW.
Firing while inverted will make you shoot very high from your upside-down perspective, which is actually shooting way low from a side perspective. The guns are sighted in to allow for gravity, firing from any angle other than right-side-up messes things up. Gravity is "reversed" while inverted, hehe.
Shooting a scoped rifle- tilting the rifle left or right a few degrees will have a nasty effect on accuracy...
MtnMan