If it hasn't been done already, I'll give Firebat and Marauder armor Ted Talk a go.
Prologue - More Than Was Bargained For;
As pretty much everyone is familiar with by now, Terran tech tends towards cheap, multipurpose, and reliable. That's no comfort to the rest of us when a solid millenium of not obliterating their infrastructure like a druggie on a bender means their standards of "cheap" are literal light years ahead, but the paradigm itself is something everybody in the Inner Sphere is familiar with. Their (in)famous CMC power armor is an iconic example, and forms the basis for two of the most respected (and hated, which is frequently the same thing) elements of the Terran arsenal - the Firebat and Marauder.
First, let's take a look at the predecessor of these two and original form of one, the CMC-200. A quick rundown of its basic features straight from the sales brochure;
-Air conditioner (a "backpack" which channels heat away from the body)
-Visor System: Lights flicker on the interior of the visor's rim acting as a basic guide. Green, yellow and presumably red feature, keeping the user informed of the suit's integrity.
-Shoulder housed ammunition reloads and sensor arrays.
-Structural support (supports the user's body, e.g. adjusts itself to fit a firing stance, improving the gunner's aim).
-Salvage System: Should the suit's arm or leg be breached, the suit seals it off in a tourniquet.
-Immunity to most small arms fire. A needle gun can penetrate the suit, however.
-Seven day endurance on internal recycling alone.
-Basic NBC (nuclear/biological/chemical) shielding.
-Limited life support.
Yeah, that's more than many Armored Combat Vehicles in a single-man package. But of particular importance to this Ted Talk is the Air Conditioner, Air Supply, and NBC shielding. See, a lot of how the Inner Sphere conducts its warfare is dependent on how well it manages heat and atmospheric conditions. A lot of places can't use VTOLs, hovercraft, and conventional aeroplanes because of the latter; and all sorts of features are limited by the ability to sink the inevitable heat produced pretty much regardless of battlefield conditions.
CMC Power Armor straight up doesn't have those problems. At least, not by any standards the Inner Sphere has ever used.
So when the Zerg and Protoss first made their appearance in the Great War that destroyed the Confederacy, that was all that was needed for the Terrans to pull the pin on the Holy Fuckery Grenade of Antioch and start coming up with the sort of wild and wacky shenanigans that soldiers dream about, engineers drink about, scientists fight about, and lawyers cry about. Cue the Firebat.
Chapter 1: I Just Want To Set The World On Fire...
"YOU'RE DAMN RIGHT IT'S DANGER CLOSE, IF YOU'D BEEN PAYING ATTENTION YOU'D NOTICE THERE'S ENOUGH BODIES TO REQUIRE UPDATING THE DAMN GEO MAPS! FIRE FIRE FI-" ~Last words of Captain Laila, Sons of Korhal Battlegroup Commander
Given the tendency of both Zerg and Protoss to use CQC troops as a core part of their lineups, something more specialized than the base model was needed. Quite a few variants were created in proper skunkworks fashion, some of which would go on to become production models in their own right such as the HERC suit which literally beats its victims to death with a reworked mining drill. But the Firebat was originally born for a different purpose - battlefield cleanup.
See, the Zerg can recover losses so easily it sounds comical at first, so they have no qualms about sacrificing hundreds or thousands to kill one of the enemy. For the Terrans, this meant that a lot of battles were lost not so much because of any particular failure, but because it's hard to shoot someone when a literal mountain of bodies is in the way. Something was needed to clear the field, and that something was the Perdition Flamethrower, with one of each mounted on the vambraces of the suit. But of course, like most Terran weaponry, it's Go Big or Go Home so this "Flamethrower" isn't the humble napalm-tosser or fusion-powered heat gun we Spheroids know and either love or hate. No, this is a damn man-portable plasma thrower that burns at multiple thousands of degrees Celsius, even more Farenheit, and Kelvin is so high it's only worth comparing in scientific testing. Anybody not in a CMC suit or equivalent who gets hit with this thing isn't going to burn; they're going to vaporize.
So of course, these Firebats were promptly up-armored and given bigger fuel tanks like the most infuriating Technical I've ever heard of and tossed into the front line. They even managed to turn a fair few battles in the Terrans' favor, though their survival rate wasn't particularly great - perils of getting within spitting distance of a swarm of monsters, who'da thunk.
Chapter 2: Terms and Conditions May Apply
"Fire is clean. It washes the dirt off the body and the sin off the soul. And my mama didn't raise no dirty boy." ~An unnamed "resocced" Confederacy Firebat.
However, it wasn't all fun and games - like many Technicals, the original Firebat had a few problems and the dark history of the Koprulu Terrans made it worse. The fuel lines connecting the tanks to the flamethrowers were only lightly armored, so a fair few Firebats were lost in large incendiary sprays when these lines were cut and the plasma sprayed out uncontrolled. The cutoff valves would kick in less than a second later, but that's far more time than is needed for wild plasma to ruin yours and everyone near you's day. Perhaps more importantly, the fuel tank had an unpleasant tendency to interact with other elements of the suit, ultimately leaking fumes into the helmet area when in combat conditions. This provoked bouts of mania, schizophrenia, paranoia, and general brain damage pretty quickly.
When paired with their extremely dangerous mission profile, almost everyone didn't actually want to be a Firebat, or even anywhere near them. So the Confederacy and Kel-Morians, and later the Dominion, did what they usually do when they want a recalcitrant rebel to do something; they cajoled, lied, drugged up, "resocced" (more on that never), and generally browbeat criminals, political enemies, and every other flavor of undesirable into wearing and fighting in these powerful but untrustworthy deathtraps. Firebats would quickly gain a reputation as insane pyromaniacs, and more than anyone wants to admit it was often a deserved label.
Chapter 3: What If We Tried More Power?
"Make it bigger. If I can't kick a Hydralisk into submission and immolate its corpse in less than a second, we're not done yet." ~Doctor Leonard Okusami, PhD
With the advent of the CMC-300 to replace the 200, engineers took this opportunity to give the ol' firebug an overhaul, mainly in the form of a purpose-built from-the-ground-up refit. In a way that makes my old bitter Spheroid heart applaud, the core chassis was fitted with a completely different outer skin that more than doubled the mass, almost doubled the size dimensions, and came to be known as the CMC-660 Firebat Package.
First and foremost, the entire fuel and containment system was overhauled. The fuel lines were buried under some of the heaviest plating on the suit, and the fuel tank itself was fitted with a variant of the famous Cold Fusion Generator to improve containment of the plasma. It is still known to fail when significantly damaged, but the fumes-to-head problem was mostly solved and the suit itself made "safe enough". With all that bulk, it is also fixes another key problem of the first-generation Firebat, being more durable than many light tanks and strong enough to flip a Mohican - much better for fist-fighting swarms of Zerglings.
But of course it wouldn't be a proper refit if the weaponry wasn't improved, and the Perdition was replaced with the Incinerator Gauntlet. A double-barreled system bigger than the average man's torso, this abomination is capable of spitting plasma flames over 100 meters through the simple physics of V=D/T. Like the fuel lines, it is thickly armored and shielded to the point where you're probably better off shooting the center mass than trying to disable the weapons for a mission kill. At least by shooting the body you might hit the helmet visor.
And of course, Raynor's Raiders have their own variation that makes the flamethrower even worse. Like most of their tech they keep it close to the chest, but what is known is that the double-nozzle design is replaced with a pair of half-domes projecting the fuel through a dozen ports each - a shining display of "quantity has a quality all its own" where that pesky Velocity equation comes in alongside the multi-directional nozzles to increase the range and kill area even more.
And just to really put the flaming cherry bomb on this volcanic cake, the Incinerator Gauntlet comes with a fire selector so it can choose which ports are open with any given shot. This was revealed during an infamous BattleROM where an unnamed firebat responded to a band of pirates taking a field trip's worth of children hostage by sweeping a pair of beer-can sized streams over their heads. Close enough to flash-boil their heads into exploding like potatos in the microwave, but leaving the shorter and lower to the ground children in their grips untouched.
Well, untouched except for some crisped nuggets and burnt hair.
Yeah, they may not be exclusively psycopaths and brainwashed robots anymore, but Firebats still have no chill. No wonder they get along so well with Reapers.
Chapter 4: Here Comes The BOOM!
"Why the hell do they keep making those spindly ass BugMechs? It's like they're begging for a kneecapping."
"Hey, I just appreciate another excuse to drink you under the table - mighty kind of them to give us some skeet. One shot per 5 knees, first to 15 wins." ~Conversation between two Marauders during the Clan Invasion of [REDACTED]
Which brings us to the Marauders. The fucking Marauders.
With the inflammatory success of the 660, further experimentation was conducted to examine what else could be done with this chassis. Most experiments ended up being too niche to put into further production, but one that was in high demand was a heavy anti-armor option that was smaller than the Diamondback and Siege Tank. Thus, the 5-4 Armored Infantry Suit.
Functionally, it's quite simple at first glance - it dumps all the fancy plasma containement and projection gear of the Firebat and uses a modified version of the AGP-2 76mm Grenade Launcher mounted on the Vulture, except instead of the classic fragmentation grenades it uses an armor piercing HEAT round. But of course it's not enough to be using a grenade almost twice as big as the classic 40mm Spheroid infantry know and love; it goes a step further and surrounds the metal core with a layer of incendiary plastic that turns into plasma upon detonation. Because just like every other blasted piece of flaming assholery the Terrans like to practice, they never use good ol' napalm when they could be using plasma instead.
To make it worse, something about the way Terrans store their ammunition gives these things a ludicrous payload. Even assuming we could fix all the other problems and make our own 5-4, we would struggle to get more than, say, 60 grenades in these things. Somehow, the Terrans can get more than 100. I don't think anybody, Spheroid or Clan, has ever actually seen a Marauder run out of ammunition.
Fun fact - it's called the 5-4 because it has 4 barrels for its guns and between them can fire up to 5 grenades per second. They usually don't need that much.
"But surely" I hear you say, "It's not actually that big a deal? Sure, it shoots harder than anything smaller than a vehicle-grade SRM, shoots faster than some autocannons, and shoots more accurately than most infantry can with rifles, but it can't be that tough right?" Well, no, it is that damn tough because just like the 660 it uses vehicle-grade Neosteel. Oh, did I not mention that earlier? Yeah, these giant gorilla suits use the same stuff Terran tanks are made out of. And if that wasn't enough, it also has a special inner lining of "kinetic foam" - not sure what that is or how it works, but apparently it's good enough that some common means of killing an armored target like overpressure and spalling don't do shit against these behemoths.
And of course, both suits are small enough to fit 4 to a dropship and 3 to a Terran Bunker. Of course they can, because it clearly wasn't enough for these assholes to be a royal terror - they had to be repeatedly air-droppable too.
Chapter 5: Imitation Is The Sincerest Form Of Flattery
"Behold, the Yokozuna! A triumph of Draconian engineering, this suit marks the next stage of Kuritan supremacy on the battlefield!" ~Dr. Jacobson Tokita, three days before Comstar notified Luthien Armor Works of a lawsuit over IP theft.
Much like all the other cool Terran Toys, everybody and their mother and their dog has attempted to beg, trade for, steal, or copy the 660 and 5-4.
Somewhat surprisingly, the Capellans were the first to come up with something roughly equivalent, that being a two-ton mini-mech with a pair of recoilless rifles they dubbed the Yanyuedao. Barely faster than basic infantry and both bigger and more cumbersome, it still carries enough armor and is small enough to serve a similar role. The Kuritans have their Yokozuna, though it only has a single smaller version of the Ultraheavy Flamer derived from the Hellion - still a pretty nasty combatant as it mounts a pair of hydraulic claws in place gauntlets. Both the Lyrans and the Davions co-developed the Chevalier which uses either SRMs or LRMs depending on the package, and predictably the Free Worlds League steals from and knocks off "is inspired by" everybody else.
Oddly enough, the 660 and 5-4 are almost unique in that there are no imitators outside the Great Houses, the Taurian Concordat, and the Magistracy of Canopus. As near as anybody can tell, they occupy a sweet spot of packing just enough tech into a small enough package that nobody else has the industry and expertise needed to make something roughly comparable.