I'm seeing a lot of issues here! Let me impart some things that I've learned!
Heavy Clip:
Nothing says "zero-spread please" like loading in heavy. I only use default ammo (as an engineer) when I'm sure I won't miss a shot.
Uncertain Angular Velocity on a Ship:
Try Zooming out. It helps me acquire a shot 90% of the time, and you can usually determine the direction of ship spin. Once you are hitting again, zoom back in for pinpointing. Or hold fire until your captain gives you a decent shot. Won't always work on Heavy guns, but I think shouting at your captain will help with this issue immensely (or them shouting at you, I like to scream 'hold fire' at my gunners when doing something fancy). I do think the gun behavior (the fact that they keep their absolute angle, rather than an angle relative to the ship) is sort of unhelpful in cases, and maybe some kind of indicator might we warranted. Something that indicates your current angle relative to the ship, and thus illustrates changes in that. Not sure how it should be implemented, maybe using the compass.
Ship Velocity:
I think this comes down to communicating with your captain again. If my gunners are on something critical like a Mercury, some captains like to call out momentum changes for them. With a gat, you can see the direction the tracers are facing, so you know when to lead forward or backwards (If the tracers are facing N and you are shooting E, start aiming S). It might not be entirely accurate, but it is consistent, and pretty easy to figure out 90% of the time.
Let me Reiterate and expand my previous post a bit:
Thinking of the projectile of the gat as a ray might help. If you are moving forward the endpoint of the ray is translated foward. This makes sense. It's weird to think about, and there may be some slight inaccuracies here.
I don't think it's so broken that it effects the effectiveness of the gat. If you think about it (and have a basic understanding of physics) it's actually very authentic. You impart your ships velocity on shots, but you also must consider the angle of the shot from its origin. You can sort of think of the motion as segmenting your angle (of course, it doesn't actually do this).
No Movement Forward Motion Reverse Leading the Target (Aiming behind)
<==| \ <==| <==|
*\ \ /*
\ <--\ <--/
\ \ /
<==| <-- <==| <-- <==|
I get that for a raycast weapon you don't really expect it to behave this way, and I do agree the tracers could be made to more agree with this behavior, but I don't think the gat itself requires a change. A point and click weapon would have zero skill index. That's not good. Not good at all!
I disagree with the sentiment that things should be easier for new players, but I like challenging games. Learning to use weapons takes time and practice. Being good at this game is 20% Skill and 80% teamwork. 20% doesn't get you very far.