Gunboats have their own versions of corsair and solaris turrets.
They are strictly for use against fighters, not other cap-ships. There are plenty of guns for gunboats to use against fighters, shield busters, basic, house, cerebrus, corsair and solaris turrets (not sure why an anti-fighter capship has battlerazors though...).
All of them work against any fighter you could ever want to shoot, if you can't hit the fighter then you either need more practice or your up against a pilot who's use to how their ship handles.
Just following the target cross icon and holding down the trigger key/button won't work on some turrets, you have to either lead the cross of trail behind it, practice against some level 18+ NPC fighters to find out.
@quintinius: I know how agile it is, sticking all solaris turrets on it will make it a ship to be careful around.
As for cruisers/destroyers:-
Cruisers/destroyers aren't scary for fighters because they aren't agile enough to be able to properly engage a player fighter.
If you go into turret mode the fighter will chew your ship up, if you go into freeflight and try using the cruiser/destroyer as a giant fighter you'll have rings run around you.
Plus cruiser/destroyer players have to gang up on a single BS player to be able to kill it off, otherwise theres no point in even bothering to try fighting a player BS on your own.
Engaging NPC cruiser/destroyer groups in a cruiser/destroyer is risky at best too.
This is because the mk 2 house VHFs with those groups can strip a cruiser shield off pretty fast.
Heres an example, the liberty cruiser group can strip the shield off and kill a player cruiser before the player ship can kill off a single NPC cruiser or charge up it's engines and run.
All because the pair of guardians and the pair of gunboat excell at taking out shields for some reason.
However, taking the guardians out at range (as they close in), then taking out the gunboats will leave you with just the three cruisers, who do bugger all to your shield.
The chimaeras and templars can do the same thing as well, so unless theres two of you after an NPC capship group you will be crippled or destroyed pretty fast.
Though to be honest it's usually easier to take out the BS groups.
This isn't a complaint before anyone asks, this is how I think the NPC capship groups/patrols should be.
Hard to impossible for a single player, easier with more players.
Try taking a BS into tau 31 when your hated by both the britonians and the kusari, I'll give your BS about five minutes, ten max, before the combined firepower of the three or four NPC BS patrols take your shield down, not to mention the hundreds of mk 2 VHFs flying around...
A single cruiser gets nuked just by two NPC BS patrols, I know this for a fact.