I love using the Deck Gun, but I leave the whole operation to the crew whilst I float towards the victim with the external cam. They're a bit inconsistant though, even stationary at close range they'll have spells just firing rounds straight into the sea or up into the sky or a km to the side.
Still, damage seems random as well, which I like, my best was the first kill on a patrol, I approached a Passenger/Cargo with a Small Merchant powering away from us on the horizon, put three rounds into it and it went up in flames then went down to the bottom of the sea
Haven't been as fortunate since