I stand by my opinion that Gunners aren't useful on light guns as much as on Heavy ones cause of the light guns not realy needing more than one ammo type and the engineer's ability to buff his weapons.
Buffed flak 4 shots a Pyra with heavy clip and you will be much more accurate and faster than a gunner using burst on it.
On Heavy guns though and on ships like the Goldfish/Spire and Galleon, you have heavy guns, you want at least one to be firing at all times, so a guy will be on his gun all the time firing it, there, I see no point to not have the extra ammo types of a gunner on it since he won't be leaving it much to repair anything and if it goes down, your secondary engi can just help him out a bit.
If they have mercuries, then better pray, cause not even 4 engineers would save your gun anyway.
Against Hwachas, if your pilot is decent and didn't manage to bring a Hwacha fish in your face, then their volley won't instantly finish off your gun, so usually, jumping off to quickly whack it once mid-volley, is a good way to keep it from going down, then you just resume firing and gump off quickly again to bring it back to full.