This spreadsheet is my attempt to answer the question, "What is the optimal configuration for each type of APS shell?".

How do I use these numbers?

Choose a sheet from the list at the bottom of the screen.
Read the headers to find the group of shells which matches your target, then choose a loader size and read down the column.
Be sure to read the header at the top of the page - it will list modules which are common to every shell (fuzes, for example).

PLEASE NOTE: Beltfed shells are calculated based on a setup of 1 loader + 1 clip + 2 intakes, giving an uptime of ~67.2%, which is reflected in DPS. Different setups will have different stats.

Are these really optimal?

They are optimal according to the metrics I've chosen, but From the Depths is a complex game, and other people may have different priorities.
If you disagree with the parameters I've used, try the optimizer, linked below. It has a lot of parameters with which to play around.

Which metrics did you use, specifically?

Shells are compared based on damage per second per marginal cost.
"Marginal" includes all the components which scale with number of intakes: the intake and loader/clip, recoil absorbers, rail chargers, and coolers.
It also includes the material cost of the shells fired, of fuel (for railgun shells), and of the ammo boxes and material storage crates needed to sustain fire for 30 minutes.
It does not include the cost of the firing piece, barrels, or gauge increasers.

Where did the numbers come from, and how can I replicate them?

My APS Shell Optimizer, which uses equations taken directly from the game code.
Source code and .zip download available on Github here: https://github.com/AoKishuba/ApsCalcUI

What if the shell I want isn't listed?

If you feel I've missed an important type, get in touch (see below "How can I get ahold of you?").
If you just want a one-off, download the .zip above (or the source code, if you have some way to run C#) and give it a try.
I've tried to make the calculator user-friendly.

Some of these damage numbers are way higher than others (cf. Frag vs HE). How can I convert between the two?

You can't. Different types of damage are applied in totally different ways, such that it's not really possible to do a 1:1 comparison between damage types.
If you want advice on which type of damage to use against a particular target, ask in the help channel on the official FtD Discord. You'll get an answer from experienced players
who know much more about the topic than I do.

What if the numbers I see ingame don't match the numbers on the sheet here?

Raw damage numbers should match the default settings ingame (for things like frag angle, HE special factor, &c).
Note that, for certain damage types, the "raw" value is modified before comparison. An example is HE damage, which I multiply by the volume of the explosion in order to
avoid generating uselessly small shells. Thus, the Raw HE row should match the value shown ingame, whereas the HE Damage row does not correspond to any ingame number.
I've tested the individual equations pretty thoroughly - and the math is open-source, linked above - so most likely you're missing a fuze or other module.
Be sure to read the header at the top of the page - it will list modules which are common to every shell.
That said, if you've triple-checked that your configuration matches the one shown on the sheet and you're still seeing an error, please let me know.

The optimal gauge for the shell I'm looking at is 500 mm. Is there any way to make my gun not sound awful?
Yes, actually. It's possible to enter gauges into the APS UI in increments of 0.01 mm, so you can use 499.99 mm gauge and get the much nicer sound.

How can I get ahold of you, O Creator?
I don't play From the Depths anymore, but I can be reached at aokishuba@gmail.com. Otherwise, leave a comment on the Github page linked above.