You should NOT be getting Mk14s on an SBoat... CapnScurvy has fixed this before... How did we go backwards again?...
The AA guns on my boat shoot straight, it's a matter of where I aim them, and what the planes' range is. When using the 20mm and 40mm single and twins, the shells will explode beyond the planes sometimes, and other times, they'll explode in front of the planes (which is better, if they drive into the shrapnel). It is advisable though, to avoid dealing with airplanes any other way than a Crash Dive!, performed in sufficient time to avoid them even being aware of your presence.
