Deadly Trail ammo Mechanic... idea:
Projectiles now have an added or changed trail that behaves similar to tar barrel. Trail is really thin at the beginning and, while staying on the battlefield for X seconds, increases to a radius of Y. Vanishing might be shrinking in radius or disappearing from origin to explosion point, something that looks pretty and is also useful in gameplay.
When enemy ships touch said trail they get damaged/set on fire/spotted, whatever. Damage/Fire would only apply to components that stand in direct contact with trail (trail touches, trail destroys).
What would be weird would be that the trail solidifies and then sinks like a ship with no balloon, or even faster (mass of trail calculated or fixed amount?). Shooting above enemies might be worth it to have them drop down and out of arcs.
Boomerang:
Projectile arcs get changed so the projectile turns around at a fixed or calculated distance.
Even if you are behind your enemy, you can still hit his front and vice versa. Might be able to shoot from one side of the ship to the other. Example: An asymetrical junker could then use both sides while it has turned its wrong side to the enemy or shoot behind your own ship.
Usefulness and Possibilities depend on implemented mechanic.
Self Destruction:
1 ammo. Fires nothing and immediately explodes, destroying the gun, upon left click.
Why? Example Goldfish: Destroy your front gun and get close without the front gun being destroyed by the enemy, rebuild gun at the right time and win.
Difference to lochnagar? Shoots nothing, thus is as stealthy as a stone (nobody suspects a stone!) (does it make explosion sound?). Gun is isntantly reloaded with default ammo upon rebuild (ability to choose another ammo?).
Sacrifice (Joke ammo?):
You pump yourself full with buffed steroids, lift your gun and throw it at the enemy. Both damage types are now impact damage (no AoE) and, of course, have massive knockback. Arcs are not so good, starts dropping at 20m? Can drop down on enemies.
Gun is lost forever until respawn.
Tar grenade: reduces clip to 1 shot, causes projectile to create 1 tar cloud on impact. (doesn't have to be ship impact) this could be used to make smoke screens by shooting at walls
Beacon rounds: Causes the bullet to light up a cloud for the duration of its life span. Does not replace flare because bullet life spans are very short.
If I look at this, I wonder if their could be harpoon-like ammo. Magnet-like ammo has already been mentioned.
In general, if there's no good way to counter something then it's generally bad design and can potentially aggravate the player.
This might be off-topic, but how do you counter ammunition or buff?