LMGs are hardly the most difficult weapon type to use, especially when gameplay boils down to deleting whatever you point your weapon at for the entirety of a playthrough.
The idea with this build was to focus on tanking early on, which gives some variety in gameplay where you have the option to leverage your resilience and health pool to beat encounters. You start with minimal perception and build it up, and later on you'll have enough to take damage-related feats to get through the harder encounters of the game.
The ultimate goal is to have a nimble tank that has just enough armour penalty to reap the rewards of Heavy Metal and Juggernaut whilst not being so encumbered that it can barely move - at level 28 I had 95% mechanical damage resistance and still 37 MP to work with in combat.
I'll post the build and then explain some of the aspects of feat order and equipment progression, as they are important to the build.
https://cannedbean29.github.io/UnderrailBuilderRemake/?HgsDAwcIBwcAwqAAAAAAAFo_AAAAwo59JT5zADcAAB4AQyvDjghfUMOdFcOew5_DoMOhAlVFw6lKwrUk4quGBd-8This build has quite a rough start. Your Guns and Heavy Guns skills will be very low due to starting with just 3 PER. Heavy Guns will get a boost at level 4 when you put a point into strength and get Brute Aim, but you will still suck with any of the pistols you'll mostly be restricted to using due to your low skill, PER and DEX.
I have enough experience in Dominating to get by levelling only Heavy Guns, but with 10/11 STR early on it is perfectly reasonable to put some points into Melee so that you can wield a spear/sledgehammer to get through the first few levels without much bother - this build is not stretched for skill points in the slightest.
Having at least 55 skill points in Psychokinesis is useful in the late game to get Telekinetic Proxy to trigger the Ambush feat, so levelling it up early will also help you along; Electrokinesis was very useful early on for taking out bots and stunning enemies in my experience.
Sneaking your way to the Brno LMG and/or crafting a Ratchet/Grudge LMG should be your top priority to really get your build going. If you are able to enter and get through the GMS compound early as well, you can loot a guaranteed Ratchet LMG from killing an enemy there, though there is no guarantee you'll get a 9mm version, which is superior to .44 in my opinion due to its lower AP cost.
Once you have your LMGs and enough ammo supply, there's not much else to say until you open the entire world map.
Once you are around level 14, you should have enough skill to craft a decent regenerative TiChrome/Super Steel armour suit with a single metal plate, so you can aim to have killed the Beast by then to unlock Leonie's upgraded merchant selection and Super Steel refinement. I wouldn't advise making more than one Super Steel plate early on, though, as you'll need a lot of charons to make your endgame armour suit.
Wearing this midgame armour will be enough to enable the Juggernaut and Heavy Metal feats.
Accessing the MG3-42 around this time is also pretty trivial with a few bear traps and some sneak skill as the enemies guarding it are bottlenecked in. It'll make good use of plentiful 7.62mm ammo that's otherwise fairly useless to you and also has the benefit of being able to burst twice per turn.
You can incrementally improve your armour suit as you level up, given that high-quality TiChrome bars are cheap and plentifully available from Leonie, and getting Armour Sloping will allow you to reinforce your suit with additional plates without ending up too immobile. Extra plates also dramatically increase skill requirement for crafting, so be wary of your Mechanics skill level before taking the feat.
I'd also recommend getting solid-padding ancient rathound striders around the midgame if you obtain a decent boot spring (high-quality leather is guaranteed if you are willing to save scum the first arena fight).
They have decent mech. damage resistance and their mech. damage reduction pairs well with Heavy Metal, Stoicism and Conditioning.
This level of protection should serve you well enough until you are ready to craft your endgame armour suit.
Feat choice at level 22 is up to you, I went with Beast of Burden because after your metal armour, two LMGs and the copious ammunition required to keep them fed, you won't have much room left to carry a lot. It's certainly just for convenience, there are more impactful feats to take still, but none of them matter too much. The build has plenty of skill points to spare if you want to, for example, pump up Intimidation and get Yell.
You'll be able to shrug off pretty much any form of conventional damage once the build is complete with your endgame gear, which includes solid-padding Super Steel striders, triple plate Super Steel regenerative armour and a soft-padding shaded Super Steel helmet. Using high enough quality plates and foam, you can reach 95% mechanical damage resistance with this setup, and getting the Nimble feat around the time you make it should be enough to keep you agile. You'll lose out on some of the damage bonus from Heavy Metal, but I think that it's a fair trade for the amount of mobility you gain (a 15 percentage point improvement translates to around a 50% increase in movement points).
At this point you'll be unkillable to most enemies in the game. Even getting bilocated and tanking all the Doppelganger's hits isn't a guaranteed death sentence thanks to your decent health pool, Stoicism and health regen from Gunner's High and your armour if you pop morphine. Get double bilocated? Cast Telekinetic Grounding on one of the Doppelgangers and move away. Once you unlock and start speccing Concentrated Fire at level 24, your killing power will become pretty nuts too. I beat the quad Naga fight at level 24 without really even trying (no drugs or mines or fancy shit, just a mis-thrown EMP nade), and that was using a Grudge made with a 125-quality frame. Concentrated Fire and W2C rounds let me kill them in 1-2 bursts each.
Not much more to say, aside from a slightly rough start, this build dominates. Being able to facetank Lunatics and other psionic freaks to an extent instead of Super Steel armour just being a crutch for low CON as is the case for a few of my builds was refreshing and made what would otherwise be a bland weapon class fun to use.
