I am certain that this will be mocked to oblivion, and I'm fully aware that this idea is a balancing nightmare. That said.
Ammo Type: Hybrid Rounds
-Effect: Combines the bonuses and penalties of all other ammunition the user has equipped
Gunners are still considered weak and limited by many people. Unlike engineers (who have almost exclusive use of most of their tools) and pilots (who are now mechanically limited to one per ship, and can use more than one tool at a time), their niche is fairly easily covered by engineers and occasionally surpassed by buff-engineers on single-ammo guns. While gunner stamina is a step in the right direction (a very good step), gunner viability is still fairly low.
This ammo type would be ammunition that is solely useful to gunners. Similar to how most engineer tools (mallet/hammer/spanner/fire tools) and pilot tools (everything but the rangefinder and spyglass) are not generally not useful outside of the class that gets them.
Certain combinations could be very interesting, some would probably be a little powerful, while others would be totally useless. It is without a doubt that some combinations would become a must-have (Charged+Greased comes to mind, as it would be +35ROF and +10% damage). This is a feature, not a bug, as this ammo would cement gunners into a must-have class for all ships without in any way diminishing engineers or changing the buff hammer.