I'd like to start off by saying that the goal of this suggestion is to improve gunner performance; but that does not mean gunners are useless. Please do not debate about the usefulness or uselessness of gunners. There's other threads for that, thank you.
The concept behind this idea is to take what I believe is one of the weaknesses of the gunner, and turn it into a strength. That is the reality that only one ammo type can be used at a time. Generally speaking, there are a few ammo types that are good for each gun, and the one you choose depends upon your preference and the situation. For instance, some gunners like to use lesmok in the gatling gun to get some early hits in, but by the time the captain has closed the distance to where you could use greased, you still have half a clip of lesmok left. You sacrificed your total damage for extra range, but now that you no long need the range, the ammo choice is actually less useful than a normal clip.
Instead of combining ammo types to give you the positive and negative effects of both, the idea is to use one ammo after another to create a somewhat separate effect from the initial ammo types. So for example you might have the flamethrower and are using greased, then switch to incendiary afterwards. This leads to a sequential combo. Instead of incendiary ammo being put in, you get 'tar ammo'. This doesn't apply the greased or incendiary effect, but a different effect. In this example it might be reducing the clip size and chance to ignite fires by 10%, but also has a 30% chance to remove the effects of chem spray per shot.
Ideally you'd need a way to decide between using a sequential ammo or a normal ammo type. I think the best way would be to have the gun be reloaded manually instead of reloading after you emptied the clip. So to choose normal greased instead of tar ammo after using incendiary, you'd empty the whole incendiary clip first. But if you wanted tar, you'd reload the gun while there's still some shots left.
We'll start with the normal ammo:
Burst
+20% clip size, -15% rate of fire, +50% aoe
Charged
-20% clip size, +30% damage, -15% rate of fire
Greased
+20% clip size, -20% damage, +60% rate of fire, -20% projectile speed, -15% gun rotation speed
Heatsink
+30% clip size, -25% damage, +50% gun rotation speed, -30% projectile speed, prevents additional stacks of fire while loaded
Heavy
-25% clip size, no spread/recoil
Incendiary
-25% clip size, -30% rate of fire, -35% projectile speed, +25% chance of ignition
Lesmok
-30% clip size, -30% gun rotation speed, +70% projectile speed, +30% projectile lift
Lochnagar
1 round, +125% damage, -90% gun rotation speed, no spread/recoil, -60% arming time, -50% aoe, heavily damages gun
((At the time of this post I am almost passing out and steam is having issues, so I can't open the game. I have used information found from other sources for the ammo types. Some things may be inaccurate, please correct me when possible.))
Initially the idea was to have a combination between each and every ammo type, but then I thought that some combinations didn't make much sense. For instance, greased and lesmok are pretty much opposite ends of the spectrum; kind of like incendiary and heatsink. So then the idea evolved into having each ammo have a combination with every other ammo, except one. But this lead to over 20 'new' ammo types, which was a bit much. Toning it down further we arrive at each ammo having a sequential combo with 2 other ammo types.
Sequential Ammo Combo's (Order is irrelevant):
Incendiary+Greased= Tar Ammo ; clip size -10%, ignition chance -10%, 30% remove chem spray buff
Incendiary+Heavy= Solar Ammo ; clip size -60%, 100% ignition chance
Lesmok+Lochnagar= Ragnarok Ammo; Single shot, secondary damage changed to impact, +200% damage, 500 gun damage, 50% of shot damage to own ship
Lesmok+Heatsink= Cooling Ammo ; -40% clip size, friendly fire, +50% to remove fire stack, 200 gun damage upon emptying clip
Charged+Burst= Thunder Ammo ; -30% projectile speed, +20% aoe, +10% damage, bright flash on hit
Charged+Greased= Super Charged Ammo; -20% clip size, +20% damage, +50% rate of fire, gun takes 100 damage after emptying clip
Heavy+Lochnagar= Big Bang Ammo; Single shot, primary damage changed to impact, no recoil/spread, +150% damage, 35% of shot damage to own ship
Heatsink+Burst= Emergency Ammo; -80% projectile speed, friendly fire, -60% clip size, puts out all fire on own ship, 40% chance to remove fire stack on hit, 200 gun damage upon emptying clip, applies 6 seconds of chem spray buff to own ship
This leads to 8 mildly crazy ammo types that can be more effective in certain situations than the regular ammo types alone. ((obviously things got weird, I'm a little tired))
The idea is for ammo types exclusive to gunners. Something they can use by using one ammo type after another, where an engineer can only bring one ammo type. Then it lets them break into the repairing and supportive roles a bit through weird ammo choices. It opens up a lot of different play styles. ((Note that none of these can be used with mines, since you can't reload the mine launcher after firing part of the clip, since it only has one shot)).
Overall I think it'd be better to get feedback on the concept than the specific suggested ideas (things got really weird there). Although I'd still love to here feedback on all of it.