When the bullet hits a block, the bullet ignites a flame in a similar way to a fire charge and light players/entities on fire.
When a firefighting "Bullet" is fired from a cannon, the bullet will damage the player/entities, while also extinguishing fires if the bullet and the fire interact.

Can promise the first, and the first half of the second. Would consider making a water bullet that puts out fires and pushes entities rather than one that damages them.

Can the water bullet both push and do slight damage

Would prefer not to be able to kill players with water. Then again, such bullets would be interchangeable with other guns that use actual ammo, so it'd allow for things like firetrucks that fire explosives. Not sure I want that...

Not wanting that.
Id rather have a “sprayer” that fires liquid particles- water or fertilizer. You dont use guns to flower your garden you know

The water bullet would be perfect, I mean MTS has several fire trucks…

While a sprayer part would visually make sense code-wise a sprayer is just a gun that shoots slow, large "bullets". Easier to just make them guns and let them use the same code rather than their own unique code.

It could easily be regulated by the caliber parameter as long as you don't accidentally make your fire hose 12.7mm or anything.

Implemented in V18.0.0.