First of all, overcharged is in the state of "go grade five or forget about it" at the moment. The main reason for that is that at lower grades it looses damage due to jitter. While the term "jitter" tells me more about the code behind it and anybody a little familiar with firearms would rather use the term "spread", that's not the main thing to consider here.
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The important part is that spread/jitter reduces accuracy and thus eliminates a lot of the damage advantage you gain unless you are very close to the target. As the gunship is not exactly considered a fast ship, you won't always manage to stay close enough to ignore the damage loss of this effect. But due to magic (or balancing oversight) the overcharge mod does not have any jitter at grade five. Thus unlike lower grades, the grade five version can project the massively increased damage output at full accuracy.
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So as long as overcharged is implemented the way it currently is, anything but grade five is a bad idea to use. Mind you, Frontier already announced to take a close look at the wreck overcharged currently is. After all it not only outperforms any other modification in terms of damage output (even those like short range blaster, which pay for additional damage by the hefty price of a massive range reduction) but also at the same time significally improves the efficiency of the weapon, not only by damage per ammo but also damage compared to distributor draw. So if you go for the effort to upgrade all your multicannons to grade 5 overcharged, take it with this more than small grain of salt: it is very likely that this modification will be reworked and will be significally nerfed. So if you invest all the work for full grade five overcharged modifications, you have to be aware that the next big patch might undo a lot of the advantage you gained from that.
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On the part of heat efficiency and incendiary rounds: while they have been a little nerfed compared to the previous version, they still are the number one experimental effect on multicannons. But the very same which i wrote on overcharged also is true for this effect: Frontier announced that it will be in focus again in the coming patches, and it very likely will be nerfed again. And on the whole heat meta: i don't have a gunship, i can't tell you how that ship now performs with a full loadout of incendiary multicannons. I guess somebody else will be able to help there, but the question also depends on what you want to use this ship for. In PvE i would not worry, i think it's still very much possible to build a ship with good enough heat efficiency to run full incendiary multicannons, if you mod your reactor accordingly. On how things develop in the PvP metagame, i could not tell you and i also think this is something which needs a little more time for people to completely figure out.
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