Jack of all crowd controls (versality chemgun + grenade launcher)

[Enchat]

Intro and theory

The idea of this build is to acquire as many crowd control abilities as possible and to play around the synergy of chemguns and grenade launchers. Between all builds I played, this one is probably the most diverse in terms of damage types, strategies and things that you can do in general. So despite the fact that usually versatility builds are not for beginners, because this game rewards narrow specialization, this one I can recommend as a means to try different playstyles and weapons.
You will have access to almost all types of crowd control in this game, the only  exception will be off-balance, but we have better means of reducing enemies evasion - acidic entanglement, which is strictly better in the majority of situations. On regular basis you will be using three weapon sets, depending on your strategy
1. Offensive. Fire chem pistol + 25 mm GL with HE grenades(incendiary when additional control is needed more than dmg)
2. Defensive. Acid pistol + 25 mm GL with frag grenades(concussive when additional control is needed more than dmg)
3. Stealth ops. Ice pistol + energy edge knife

Plus plasma pistols for bosses or robots and shock shurikens for finishing off wounded enemies. Plus access to three psi-schools. This build also heavily relies on crafting, and has 10 int, which gives us access to insanely overpowered Specialist feat and also allows to craft all the high end stuff, that now have requirements of 200+ in their respectable skill.

Problems
As downsides, I can mention:
1 The build is weak against oneshotting ranged enemies. Snipers will be your nemesis, they outrange you because the majority of your crowd control works on short to middle distances. They outdamage you because you are doing damage with dots effects. And with 3 con you will be often oneshotted by their aimed shots. And with 3 agility and debuff from backfiring time contraction you often will be unable to LOS them. There are still some counter play available, stasis despite the recent nerf still remains major boost to survivability, flashbang with our high throwing, and prep+electrokinesis

2 Lack of carrying capacity and hence the required amount of inventory management is quite a problem. Ammunition for both chem guns and GL weights a lot, and you have only 5 str. You are also able to craft all MK5 grenades, plasma ones, and naturally you will be tempted to carry at least some of them. This problem  can be to some degree circumvented by taking pack rathound, and this build have spare feats point, but doing so without hamper combat efficiency won't be possible until 22 lvl

3 Chem guns and grenade launchers have a major conflict in terms of using belt slots. Playing chem pistols you don't have a choice but to use catalysing belts, it is just a mandatory, they simply don't work without it. But being low on str you have only access to 6-rounds magazine in your GL(or 1 round if you prefer 40mm Thumper) and there will often situations where you have empty GL but cant spare 30 AP to reload it.

The build:

Ok. The build itself. The character builder is a bit outdated, so I’ve used Expertise as a substitute for Polymath, and Ninja Looter for Specialist. There also will be 150 additional skill points, and 15 specialisation points that can be spent according to your preferences.

https://underrail.info/build/?HgUQAwMGAwojAMKgAMKgAAAyMigAAGRkZGRkAEYAwqAAADIrT8KjASYqVsKRAsKHJS9RFjHCs-KouwPisrsF4rWuB9-8

After you take Fatal throw at 20th level the build can be considered finished, and the last two feats before Increased Dexterity are optional. Apart from Grenadier and Recklessness you may consider:

• Pack Rathound - probably the best QoL feat for us
• Paranoia - always nice to have
• Escape Artist - though I prefer to spend 70 skill points on Exothermic Aura, witch gives us additional area denial capabilities
• High-Technalities - aimed shots from plasma pistol, then follow up with entropic recurrence is a boss killer
• Steadfast Aim - being fully specced it is ~10% increase in dps of chem guns
• Execute - looks good on paper, but life of the enemies who is close to us and incapacitated is already miserable to extreme

Itemisation
Crit googles of head slot
Psionic Strudy Vest with Antithermic Overcoat, strudy here is much better than regen one, despite the build is playing around dot effects, crowd control and prefere prologed fights in general, our main cause of death always would be oneshots from snipers or psionic. Playing this build I seldom died of attrition like tanks do.
Advanced Catalyzing Belt - no brainer
Metal boots with spring. Caltrops remain relevant till the endgame, because we have an ability to make enemies pay for every cell they advance

Start of the game:
The build was created with dominating oddity run in mind, and despite not being as beginner friendly as something like optimized AR tin-can builds, I still think that this is a great build for people who just started playing this game, because of the sheer diversity of the toolkit. Apart from your main weapons you will be occasionally using melee, sniper rifles, all the utilities, throwing weapons, you can craft all the pharmacology and use majority of psi abilities.

1. Rathounds. Take 1 lvl - Polymath+Opportunist. We are starting as a generic psi-knife build. Take a crossbow as your weapon of choice in the armory and buy the best knife they have.
Take pill on med floor, and borrow all the medical consumables
On the agriculture floor buy 1 molotov, take flares and nets from the store room and get a quest for rathound tissue from a methatermic trainer.
On the engineering floor steal a free battery, haxor and buy a few lockpicks.
Head to the top flor, the one with the metro station,  learn stun from Bison, head to north and take oddity from barrels near the entrance.
Then go  to the caves, first visit the area with Mordre and cave hoppers.  Keep murdering those little cuties until they drop the oddity
After that you can proceed with rathounds. The combo of stun+opportunist allows you to kill lone ones, for groups use flares and kite them to fences where you can close the doors and shoot them with a crossbow. Molotov+stun kill crossbow guy. If you play this right you would take any damage here.

2. Psi beetles. This is the hardest part of the game for you, I'm not joking. You need as much preparation as possible. Get some tailoring and 20 chemistry, buy recycle item and incendiary grenade blueprints, from area with rathounds you should get a blueprint for leather armor, craft as many armors as you could and then recycle it for Dirty Rags, check all barrels in the vicinity of the station and look for bottles. Then buy gasoline and craft 5-7 molotovs. Learn Force Field and Psycho-temporal Dilation. Take nets from the store room on the agriculture floor. If you have spare money - craft taser.
The tactics with single beetle - come close in stealth, use temporal dilation and start stabbing. It should spend two turns on just trying to break the distance, without attacking you. Then use a stun+aimed shot and try to finish it.
Groups are much harder, use molotovs, and try to separate them with force field, use throwing nets, and being rooted in melee they switch to melee mode, which is far weaker. In general you just need luck here, lucky crits, good molotov throws and etc

3. Robots. Before you go to omega station, try to sneak to junkyard and take all the oddities here, this will allow you to have prep+electrokinesis combo, that with some hard hitting pistol trivialize this encounter. Try to craft several gas grenades and kill Mordre with them, for some reason he is not reacting to them. Seems like this man has a subconscious death wish because of guilt or something.

4. Depot A. This is where craft is rising to power. You need poisoned caltrops, metal boots, armor with acid protection(first siphoner than mutedog), fire chemgun, sniper rifle and MK3 grenades. This is usually enough to discourage dogs from even approaching, so they will be only spiting from distance. The only problem is Mutants, they can be netted, stunned and flashbanged. You can use Force Field to prevent dogs from rooting you and leave the map for healing, reloading or changing entrance.

5. Burrowers and Rathound king.  Telekinetic Proxy helps to trigger ambush and clear spawns. Craft plasma pistol for burrower warriors and ancient rathounds. Both Telekinetic Grounding and Exothermic Aura  is extremely strong if you able get them

After that there should not be significant problems. Beast fight is kind of free win for acid gun

[Enchat]

Ok, after finishing the game with this build I've several more things to add

Additional feats:
Point shot - garbage, extremely unreliable, at the end of the game we will have 10 AP attack for chemguns, this ability allows us to save 5 AP for, but it miss oftener than it hits, and its exacerbating the late game problem of chempistols - 8 rounds magazine and constant need for reloading. Although if you are going to take High technicalities and carrying smart plasma/electroshock pistols - this maybe of some value

Execute - it works much better than previous one, but still it is hard to use it because of how many conditions need to be fulfilled, the hardest one - close range for 3 agi build.

Steadfast Aim - the value depends on many factors, but according to my calculations its about ~7.6% dps increase for high-end chemguns. The same amount you will get from high quality infused rathound leather armor. Much better that two previous options

Future orientation - on paper it doesn't sound like a big deal, but in practice it makes a big difference. With it, the uptime of temporal contraction will raise to almost 100%, and it increases the amount of cooked shots - our main source of damage and control. Highly recommended, it should be in all chemgun builds

High technicalities - With this feast at the end of the game smart plasma pistols deals ~1500 dmg with aimed shots. It allows us to reliably snipe down priority targets. It also helps on stealth missions and pairs well with ice guns.

Fatal throw - works at maximum efficiency in the middle game, but damage of chemguns grows faster than hp pools of enemies. Acidic entailments from high end pistols occasionally tics for 300-500 damage, which is often enough to skip 25% of the window entirely. I still like this ability, and in this game the further you progress the easier the game.



Items:
Here we have basically two approaches - area denial or stealth+mobility.
First one implies heavy use of incendiary grenades and poisoned caltrops, hence we need antithermic overcoats and metal boots, it is a defensive style where you want to provoke enemies to attack you in some enclosed space and then hold the line.
The second approach is  more offensive, it allows you to scout enemy positions in stealth and dispose of isolated targets with an ice gun + plasma. For items it utilizes leather armor and tabi boots. Armor either is infused cave hopper or infused rathound, i prefer the cave hoppers because our main weapon have 4 effective range and we have 3 agi. Infused rathound armor gives like 11-12% crit chance, in theory this means 7% increase for damage of chemguns, that not much, and in practice it even less because with ambush and focus stim crit chance easily reach 100% anyway, and you constantly use aimed shots. For boots are wider but less impactful, I've played with ninja, but an infused mutated dog or infused  siphoner looks good as well.
Unlike feats choice we can change approach anytime, in my experience the further game progresses the stronger offensive style becomes, so I suggest to start with first approach and convert to second when you finish beast quest and gather all super steel refinery data.

Guns:
Our main source of damage - acid pistols. I've seen some other chemgun builds that suggest that fire pistols outdamage them. It is understandable why such opinions have a place, in attach characteristics of high-end chemguns, acid DoT have  60% chance for 351% damage and it doesn't stack,  fire DoT on the other looks much better with 75% chance for 645% and it stack with itself. But the problem here is the inexplicit mechanic of mad chemist feat, it works differently for each gun: for acid it increases damage per round, as expected, but for fire one it increases the length of the DoT, damage per rounds remains the same. That means that this big numbers of fire pistols are indeed big only in prolonged fights. I did calculations for high-end weapons and according to it acid gun significantly out-damaged the fire one in the first 3 rounds, around the 4th fire one catches up and at 5th and further starts to lead. But the thing is that nothing except a maybe final boss lives that long.
There are also additional factors that i didn't include in this calculation, like acid trap proc opportunist, and fire dot set up ambush. This may make some fluctuations in damage charts, but under simplified conditions in terms of pure damage acid gun killing stuff almost  twice as fast than fire.

Ice gun is not for damage, it is still invaluable for its controlling capabilities. Although in the first round before dots tick it still outdamage other contenders.

Conclusion - acid for dealing damage, ice for stealth, fire for control and for some root immune enemies like coil spiders.

Additional weapons:
Grenade launcher to soften targets from afar, additional aoe, killing psionics with mirror images. Despite zero feats investments damage is still good, especially on targets in acidic trap
Plasma pistol - alpha strike, stealth assassinations. Helps to conserve acid on naga and industrial robots.
Additionally i've tried sniper rifles, knifes and pneumatic gloves. While not being completely useless there still better contenders for weapon slot

Specialization:
Specialist feat is a big deal for this build, because now we are able to full spec both versatility and mad chemist. The latter is often overlooked, I've seen builds that take opportunist spec instead of it, this is a mistake. I did the math and each spec point of mad chemist giving us about a 3.5% increase of damage(calculated it for acid, for fire it will yield more), but its works despite the condition of a target, so in the majority of situations each point spent on mad chemist yields more damage. An exception is the ice gun, mad chemist spec only increases slow duration, so my final ice pistol slows targets for 16 turns.
We have 3 specializations that clearly stand out from the rest: versatility, mad chemist and cooked shot cost, they will take 25 points. The rest 5 are not that important, and maybe spent according to personal preferences. I've taken fatal throw AP and premed CD reduction


I would be very grateful for any feedback, and feel to ask any questions about chemguns.