Bullets of different calibers have different trajectories, meaning a shot that works for the 7mm's doesn't work for the 20mm's, resulting in quite a few bullets missing. The way around is possible too, 20mm's hitting but 7mm overshooting. 7mm tracers give false indications for leading, and seeing a lot of hit sprites you could think that you hit a lot but the enemy is undamaged.
Also having all that 7mm gives ideas of shooting 800+ out when the enemy is extending, which is very bad of course, because I don't think that even 1% of these bullets will ever hit anything (or you are a god).
Score wise in a spit I'd say not the fire the MG's, or put convergence on 200 on all guns and stick to close range shots.
That's partially true, but the trajectory differences take too much blame IMO (which leads to the constant, ineffectual, tweaking in an effort to "fix" the problem).
When firing at convergence distances (with the various guns set to the same convergence) trajectory makes no difference at all. At convergence, it doesn't matter if the various guns exhibit identical trajectories, or vastly different trajectories; they'll all shoot to the same place (but won't get there at the same time).
The problems come into play as soon as there is any apparent target motion to deal with. The reason once again has less to do with trajectory, and more to do with
time. A cannon round and a MG round, fired at the same time and going to the same place, won't get there at the same time.
The result is that a different amount of lead is required for each gun. The cannon rounds (being slower) require more lead. Essentially, it means that if your getting hits with more MG's on a crossing target, your lead is wrong to get hits with your cannons, and vice versa. On a target that appears stationary (saddled up on an unsuspecting target, for example) that time difference doesn't matter, but on a crossing target it could mean missing in front of a target with your MG's (too much lead) while also missing behind your target with your cannons (not enough lead) on the same pull of the trigger.
Beyond that, if the pilot is pulling anything above 1G (i.e if he's turning) while he's firing he's going to exaggerate this effect. This comes into play if he's trying to minimize apparent target motion by following his target through a turn.
At anything beyond convergence (as in your example) trajectory (which is a factor of time) comes into play. But at that point it's really just taking a bad situation and making it worse.
From a Hit% viewpoint, firing at crossing targets is always bad. It's worse if you have mixed guns. In both cases though, firing at a crossing target is ok if you fire in short bursts with the correct lead (and proper aim). If you fire a 1 second burst there's always going to be "waste" on a crossing shot. There's really only a small window within that 1 second burst where hits are going to be possible (with either gun). If it's a .25 second window, that means that .75 seconds of your 1 second burst are guaranteed misses (even with absolutely perfect aim). This takes your potential Hit% down to 25% already.
But being bad from a Hit% viewpoint doesn't mean they're bad shots to take, it just means you want to limit them to
very short bursts (my bursts are tiny on these shots; I view them almost as if I was firing a single-shot weapon rather than MG's. When I flew CHogs a lot I was generally only firing 10-12 rounds in a burst). Even that was a lot of waste, since I could generally only expect 1-3 rounds to hit my opponent in any one burst (less than 10% hits). I actually find these are some of the best shots for me to take though (as long as I can hit with them) because even those little bursts will pick an opponent apart and demoralize him.
When it comes to a "mixed gun handicap" on these types of shots though, I'm not sure there really is one. For starters, less cannon rounds are being fired, so have less effect on your hit% (even if they all miss) than the multitude of MG rounds. Also, the cannon rounds will have more effect then the MG's if they do hit.
