I use multicannons as the primary weapons on all of my ships, all with grade 5 overcharged rounds and either incendiary or corrosive experimental effects so I can probably help with some of the advantages and disadvantages of either weapon option.
So trying to figure out the best type of experimental (if any) for my 4x2F Multi-cannons.
All the MC's have G5 Overcharged to them. One of them has Corrosive Shells and want to know what to use for the other 3.
I've heard that corrosive doesn't stack so there's no point in wasting that on the other MC's. Is that true? If so, would it be useful to use Incendiary Rounds on the others? Or should I just leave the other MC's as is without any experimental effects on them?
For PVE combat the choice is really between corrosive or incendiary. Emissive rounds are useful for PVP to stop players from disappearing by going into stealth mode, but NPCs don't use stealth, and the other MC round effects are not particularly useful to have on a single weapon (i.e., auto-loader, smart rounds, etc. don't help much on individual weapons unless your entire build is based around them).
In terms of deciding between incendiary and corrosive, I would think of them almost like completely separate weapons.
Incendiary multicannons are essentially like plasma repeaters, they inflict thermal damage and generate high heat. They're very useful against shields and less useful against hull compared to regular multicannons. The main drawback is that they have 3X normal heat gemeration (shown in outfitting as a 200% thermal load increase). I generally try to use 2/3 of my multicannon loadout with incendiary rounds on most ships to ensure that I can take shields down effectively.
Corrosive muulticannons inflict kinetic damage and are better than incendiary rounds against hulls, plus they generate a corrosive effect that increases hull damage to the affected ship by 25%. The corrosive effect lasts 5 seconds and doesn't stack, which is why many players only use a single corrosive. The drawback to corrosive rounds is that ammo capacity is decreased by 20% from 2100 rounds to 1680 rounds. I generally try to use 1/3 of my multicannon loadout with corrosive rounds on most ships. If I only have a single large or huge hardpoint (such as the Diamondback Explorer or FDL) then I usually put the corrosive rounds on the largest multicannon to ensure that I don't lose the kinetic hull damage against large ships.
In terms of optimal loadout, most players will suggest a single corrosive multicannon and the rest with incendiary rounds, but there are a few issues where this won't always be optimal. First, you want to ensure that you keep enough corrosive multicannons that you can still do enough hull damage and on some ships this might involve using more than one corrosive multicannon. This is really just to preserve the kinetic hull damage (since the corrosive effect itself doesn't stack) but other than the ammo capacity reduction there's no other drawback to the corrosive rounds so in most cases you might as well add the effect if you're not using incendiary rounds (plus you can chose to fire corrosive multicannons only once shields are down which generally makes up for their lower ammo capacity). The second issue is that on some ships using widely-spaced or poorly-positioned hardpoints for corrosive multicannons can run into issues with convergence or field of fire where the weapon is only firing part of the time. This means you will be potentially losing that 25% hull damage bonus and having a second corrosive multicannon to improve the firing arc means you will always have at least one multicannon with corrosive rounds able to hit the target. The third issue is that FD has said that they want to re-work how corrosive weapons work to make it beneficial to have more than one corrosive weapon. They haven't given any timeline or details for this but eventually it might be beneficial to have multiple corrosive weapons and this is another good reason to incorporate more than one of them into your weapon loadout.
I've also read that Incendiary Rounds got the ax in previous updates, so as of 2.3, are they worth it? Plus in older posts they state they raise your own heat, but the description in the actual game doesn't say anything other than it increases damage and converts a large portion to thermal which I assume is against the targeted ship.
I use incendiary multicannons as the primary weapon on all of my ships so I have followed the changes quite closely since incendiary rounds were introduced. I usually combine them with corrosive rounds but I do have some ships, such as my Vulture, which are armed entirely with overcharged incendiary multicannons as that happens to fit my overall loadout best. The incendiary rounds (as well as the overcharged mods themselves) went through a number of nerfs which are rather complicated, but I can summarize all the changes for you.
Incendiary rounds when originally introduced in 2.1 were quite useful when combined with the overcharged mod because they converted most of the damage to thermal but also retained an additional 20% kinetic damage. Effectively they had 20% "bonus" kinetic damage compared to regular multicannons which offset much of the reduced hull damage from being converted to primarily thermal damage. This meant they were good against shields but didn't actually lose much hull damage as the extra 20% kinetic damage made up for much of the thermal vs. kinetic drawback against hulls. This original version had no effect on increasing heat generation and this is the factor that made them rather OP compared to other weapon options. This original version, however, only lasted a short period of time from 2.1 and 2.1.02 before being dramatically nerfed by FD shortly afterwords. They were essentially seen as being the most effective weapon overall as they could generate high shield damage as well as good hull damage for minimal thermal load and capacitor draw.
The second version of incendiary rounds, i.e., the first incendiary nerf, was introduced in the 2.1.03 patch and involved increasing the heat by 3X (200% increase) and decreasing the ROF by 10%. Essentially they still inflicted significant shield and hull damage (despite the slightly 10% ROF reduction) but their use was now limited primarily by heat buildup. This was a substantial issue for many ships as the ROF was still high enough that it could overheat some ships rather quickly. The overall dps vs. the original version of the weapons however was only reduced by around 10% which was due entirely to the ROF reduction and they remained effective weapons against both shields and hulls despite the reduced ROF. This was probably the most "balanced" version of the incendiaries as they remained powerful weapons but required builds that could deal with the high thermal load. Subsequent versions of incendiaries were nerfed to make them less effective weapons overall (especially against hulls) as part of the overall goal of weapon "rebalancing" that occurred during the 2.2 patch.
The third version of incendiary rounds, i.e., the second incendiary nerf, was introduced in the 2.2 patch and involved removing the additional 20% kinetic damage (that it originally received as "bonus" damage compared to regular multicannons) and improving the ROF from a 10% penalty to a 5% penalty. Essentially this reduced the overall damage by around 15% again compared to the nerf in 2.1.03. In this case the dps reduction was due to reducing the extra 20% kinetic damage and was slightly offset by improving the ROF by 5%, which overall meant that the damage was around 15% less overall. The incendiary rounds were still an effective weapon but now were much less effective against hulls. They had essentially become specialized as thermal weapons at this point making them function much like plasma repeaters overall due to the loss of the kinetic damage against hulls.
The fourth version of incendiary rounds, i.e., the third incendairy nerf, occurred with the 2.2.03 patch. It wasn't actually a direct nerf to incendiaries but was rather a dramatic nerf to the overcharged mods that incendiary rounds were generally used in combination with. This involved a slight increase in damage of 5% to the overcharged mod blueprint but also a complete removal of the ROF increase from the blueprint which had previously contributed around 15-20% of the overcharged dps bonus. The incendiary rounds however still retained their 5% ROF penalty (relative to normal rounds) and were hit particularly hard by the loss of the overcharged ROF bonus. There was also a variable clip size penalty applied to overcharged mods as well. Effectively this combination of changes decreased the dps of incendiary rounds by another 15% overall when applied to overcharged mods. The removal of the ROF increase for overcharged mods had the slight benefit of making the heat from incendiaries somewhat more manageable but the overall dps reduction also made incendiary rounds even less useful against hulls.
It's worth mentioning that corrosive rounds were also hit by the effects of the overcharged blueprint nerf in 2.2.03 as well, but since most builds only included one or possibly two corrosive multicannons the impact was less noticeable on those weapons compared to the impact it had on incendiary rounds. There's also the issue that the new rapid fire mods remained a possibility for re-rolling corrosive multicannons but were very suboptimal for incendiaries due to the impact that relying on a ROF-based blueprint had on heat generation, not to mention that the changes to heat mechanics and the increase in module damage from overheating made it even more important to be able to manage the increased heat from incendiary rounds. You could possibly consider the heat mechanic changes a "fourth incendiary nerf" but only indirectly as the heat changes really affected SCB use more than they impacted incendiary rounds (especially since you can simply stop firing temporarily to reduce heat buildup but can't stop a SCB once you're triggered it).
If you go overall all of the incendiary round changes and compare their dps when originally introduced they went down a total of around 35% in terms of their dps from their introduction in 2.1 to the most recent nerf in 2.2.03. This occurred in a series of stages where they first lost 10% ROF, then 20% kinetic damage, then regained 5% ROF. They finally lost a massive 15-20% ROF and gained a slight increase of 5% to base damage from the overcharged mod changes. Overall that makes them around 65% less effective then when originally introduced. They are still useful weapons but compared to corrosive rounds, which retain full damage against hulls, plus have 5% better ROF and the corrosive 25% hull damage bonus, they are really primarily useful against shields instead of being a true "multipurpose" weapon at this point.
