I don't play as much as I would like, and I am sure there are a lot of new players who are looking for answers. So lets compile a list of the best ammo types for each Gun.
Whirlwind Gatling Gun.Ammo: Charged
Reason: Best all round for increased DPS while still effective at medium range
Ammo: Greased
Reason: Best DPS but only at short range due to increased aim giggle.
Mercury Field Gun.Ammo: Charged
Reason: Increases Dammage while maintaing 2 shot clip size
Scylla, Double-Barreled Mortar.Ammo: Lesmok
Reason: Compensates for slow projectile speed and increase odds of hitting fast ships at medium range.
Ammo: Greased
Reason: allows for maximum DPS for the brief window when the armor is down.
B
arking Dog, Light Carronade.Ammo: Heavy
Reason: Removes shotgun scatter and ensures all pellets hit the balloon.
Ammo: Incendiary
Reason: Gives each pellet a % chance of starting a fire, scatter effect causes a lot of fires in different parts of the ship.
Banshee, Light Rocket Carousel.Ammo: Greased
Reason: This gun is used as much for the ignite chance as the explosive damage. More shots fired means more ignite chances. Range and accuracy is already high on the weapon so Greased Round's negatives do not hinder the weapon performance much.
Dragon Tongue, FlamethrowerAmmo: Lesmok
Reason: Extends range of flame thrower if the captain can't get you close enough
Ammo: Greased
Reason: Sets fires faster but shortens range. Directly targeting weapons with greased flame can induce flame lock in a few seconds.
Please post with the weapons I missed or corrections to the weapons I included. As I said, my knowledge of the meta is a bit out of date. See Echoez's guide, which is a bit out of data for more answers.
https://gunsoficarus.com/community/forum/index.php?topic=1628.0