TL;DR at the bottom. Yeah, I'll make you at least scroll through all this, because it took quite some time to write.

So I've noticed that whenever there's a discussion about secondary accuracy, in particular German secondaries, almost no one knows the real differences between pre 10.0 and post 10.0 values. Whether it comes down to simply referencing some misinformed values, not knowing or remembering the right percentage values, or not knowing how to apply the math, I want to try and correct those misconceptions with this post. For the upcoming calculations, I will use values found in

wowsft.com/ship.

I will show the differences as pure dispersion multiplier values, as well as compare three different secondary arrangements: Großer Kurfürst (current), Republique (current), and Republique/Großer Kurfürst (pre-rework). The last one is actually just most BB's before the rework. From now on, Großer Kurfürst will be referred to as GK and Republique as Repu. I don't have the exact stats for the pre-rework Repu/GK, but we know that the base dispersion curve is the same as Repu (along with most BB's) currently has, and while the range is different, that does not really affect the accuracy calculations. Actually quite contrary, it's easier to compare accuracy at max rage when we can use the same max range for all ships. I chose Repu in particular because it's the only ship that has the same secondary range as GK, but the old dispersion.

So first, let's see what has actually changed. The obvious change of course is that while the old Manual Secondaries (ManSec) had 60% dispersion reduction, the new Improves Secondary Battery Aiming (ISBA) only has 35%. Another change is that German BB's got an inherent dispersion reduction somewhere between 22% and 22.5%. This can be proven by comparing any French BB's secondary dispersion at max range with the same tier German BB's secondary dispersion at max range. For example, Repu has 504 m at the base 8.3 km max range, while GK has 392 m. 392/504 = 0.7777…, which means 22.22…% reduction. Because the values are rounded to full metres, I can't get a consistent value, but for the benefit of the doubt, I'll use 22.5%, which I've also heard being mentioned by someone else before.

All other bonuses to secondary accuracy have remained the same. Secondary Battery Modification 1 gives 20% reduction, Aiming System Modification 1 gives 5% reduction, and Mike Yankee Soxisix gives 5% reduction. The first two are mutually exclusive, so we will disregard the latter one from now on.

At first someone might think that "hey, 35+22.5 = 57.5, so the nerf is only 2.5%". However, this doesn't work because the dispersion reductions are not added, but multiplied. So the correct calculation is 1-((1-0.35)*(1-0,225)) = 0.496, so about a 50% combined reduction in dispersion. So here we have the most common misconception, that the accuracy reduction is 10%, because that's the difference in reduction percentages. However, comparing reduction percentages doesn't tell the whole story, especially with reductions this big, but we have to compare the final values instead.

In the chart below, we have all the best possible reduction percentages, and the total dispersion multipliers for each configuration.

Summary of all the secondary dispersion bonuses

From the total dispersion multipliers we can calculate the true difference in the dispersion. Secondary Mod 1 and Mike Yankee Soxisix were not necessary for this calculation and would not change the final difference in percents, but I included them since they would be there for any full secondary build. So the real difference in dispersion between old GK and new GK is:

0.383/0.304 = 1.26

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So the new German BB dispersion is 26% worse than the old. The dispersion difference for Repu (and most other BB's) is:

0.494/0.304 = 1.63

This means that overall, ISBE has a 63% dispersion radius nerf compared to the old ManSec. Not 25%, as it would seem at first glance. These calculations with dispersion multipliers might seem a bit abstract, so here is a chart with actual dispersion values at the current base range of 8.3 km.

Calculations for actual dispersion values at 8.3 km

In this chart, the 22.5% reduction for GK is already in the base dispersion value of 392 m, so it's not multiplied in again. You can see that the differences for full secondary builds are 194/153 ≈ 1.26 and 249/153 ≈ 1.63 when comparing the new GK and Repu to the old ones.

However, the reality of how badly the dispersions were nerfed doesn't end there. We usually think of the dispersion changes as only a percentage change in horizontal dispersion, but there is the vertical dispersion as well. We know from this post that even though vertical dispersion doesn't scale consistently, its changes are directly proportional to the changes in horizontal dispersion at any given range. We also learn from that post that, quite oddly, horizontal dispersion at any given range gets better when max range increases, so if that is also true for secondaries, the range buff they got is also a slight vertical dispersion buff. However, I don't know the exact effect, and the change should be so minimal that I think we can disregard it for the sake of simplicity.

So, as GK has 1.26 times bigger horizontal dispersion than it used to, it also has 1.26 times bigger vertical dispersion. This means that actually the area that the shells hit, is 1.26^2 = 1.59 times as big than before the rework. For Repu and most other BB's, it is consequently 1.63^2 = 2.64 times as big. So, in reality, accuracy is 59% worse for Germans, and 164% worse for most others. This is somewhat in line with practical tests that some people conducted by measuring the time it took to kill a stationary target with only secondaries pre and post rework.

(This is only to provide a bit of contrast, so don't take this comparison too seriously. Changes in secondary accuracy do not have the same effects on the meta as changes in main gun accuracy.) Deadeye, which is big enough of a buff to completely twist the meta, gives your main guns 10% dispersion buff, which translates to 19% accuracy buff. In comparison to the changes in secondaries, this change is quite minuscule. That's how badly secondaries got nerfed.

Finally, one might ask that what this means for the American premiums with improved secondary accuracy. Well, as far as I know, they have 40% better dispersion by default, the same they used to have. So, in percentages, their dispersion when using ManSec/ISBE was nerfed just as much as most other BB's. Quite curiously, their accuracy with ISBE is now approximately the same as most other BB's had with ManSec before the rework. So while in theory they were nerfed as heavily as most others, they still remain usable. Though nowhere near as great as they used to be with a full sec build.

TL;DR:

German secondaries with full sec build have 26% worse dispersion than they used to before rework, and that translates to 59% worse accuracy, at least in theory. So please stop with the "it's only 10% nerf, it's not a big difference". It's totally a huge difference.

Most other secondaries with full sec build have 63% worse dispersion than they used to, which translates to a theoretically 164% worse accuracy. This means that secondary build is practically unusable for any ship that doesn't have an improved accuracy by default.