"Emperor protect my soul, ﬂak protect the rest. "
- – Common Imperial Guard saying
Flak Armour (aka T-Shirts) is the most common form of Armor employed by the Imperium of Man and especially the Imperial Guard. Cheap and easy to make and manufacture by the average Imperial world, it is composed of multiple layers of ablative material, shock resistant material and ballistic textiles. Some layers commonly used include Carbon-fibre, Plasfibre and Thermoplas strips, although a number of specialised materials can be integrated in non-standard suits.
A flak vest consists of a skeleton of lightweight, flexible metal. This skeleton is then wrapped in multiple layers of a high-tensile fabric that is the main protective component of the armour. After multiple layers of fabric are affixed to the skeleton,the vest is given its toughened outer shell. The same principle is used in the production of Guardsman helmets and bracers. Guards for knees and legs are also produced. Thermal-absorbent materials, applied to reduce the thermal signature of the infantry, help with staying invisible during night recon missions. Rarer still, Cameleoline is incorporated into the outermost layer of some Imperial Guard Regiments' fatigues providing yet more concealment.
Fairly similar to modern body armor in regards of rough construction, though the materials are more advanced. It is resistant to shrapnel and stab attacks done by human combatants armed with regular knife while offering pretty good protection against bullets from combustion guns and glancing (or direct, lore varies) hits from directed energy weapons comparable in power to a Lasgun. Might sound unimpressive but keep in mind that it can stop anything short of an anti-vehicle weapon like anything with the word "heavy" or "missile" in its name. Bolters, too, which are basically dramatically larger versions of modern anti-tank rifles with rocket propelled explosive bullets.
How Good Is It?
For example, modern Earth ballistic vests, constructed from Aramids like Kevlar, are either light weight vests that provide reasonable protection only against shrapnel and weaker rounds, such as .45 and 9mm, up to .357 and .44 Magnum or poor penetrating (will stop a shotgun slug but will result cracked and broken ribs at most, bruising like Mike Tyson punched you at the least), or heavy ballistic vests that equip heavier hard armour inserts that provide more coverage and withstand more powerful rounds, all the way up to armour piercing sniper rounds (at 7.62 mm / .30-06 caliber), assuming the wearer has some sort of metal or ceramic insert and can endure 3-12 shots from modern assault rifles before shattering depending on generation). Even then, modern body armor, composed of ceramic, titanium, and ballistic fiber at its finest, is useless against heavy machine gun to autocannon caliber ammunition (12.7 mm / .50 BMG and above) with, at best, a glancing hit to the best armored points "only" shattering bones rather than blowing limbs off.
According to lore, Flak armour gives a chance to not only survive a couple of impacts from a light machine gun or several from autoguns, but the wearer remains able to continue fighting (assuming it hits the chest piece), all while not weighing down the soldier significantly. When dealing with a local rebellion or facing pirates, bandits, hive-gangsters or the rag tag forces of the lost and the damned, a suit of flak armour is an Emperor-send and often means the difference between survival and a gruesome death. After all, it's said that any stubber at and below rifle caliber will (supposedly) not pierce any part of the Flak Armour. So, in theory, Flak Armor makes you much harder to cut down, and fluffwise, it has also been said a laspistol will not defeat the chest armor. There are two thing to note: First Stubbers are THE most common type of weapon in the entire galaxy. Their ease of construction and maintenance means that they are the most likely weapons the Guard would most likely face. Flak Armour is known to deflect Autogun rounds (8.25mm bullets ("8mm Mauser" used 8.22mm bullets)) from a certain angle via the sloping effect, so it would most likely tank a direct hit of the much smaller bullets chambered to fire in primitive stubbers. The second is that of course lasguns are not ballistic weapon but energy weapons, so the requirement's to "deflect" a lasgun shot are different from that of a conventional weapon. That it can do both feats at once is no small accomplishment. (
Though it is noted to only stop glancing hits from a Lasgun - a laspistol is significantly lower power than your average Lasgun lore has both glancing and direct hits, likely depending on pattern of both lasgun and armor, pistols in the lore have the same power as the full gun but shorter range, which is why they are mostly reserved for officers as that's very advanced tech.) However, it may be that the angle that it is hit at simply gives enough protection to block a shot.
Note that autoguns are to stubbers what 21st century guns are to WW2 small arms (and can be anything from caseless ammo to rail or gauss automatic weapons to rifles using gravity manipulating technology) and stubbers other than handcannons and heavy stubbers are outright incapable of penetrating flak armor at any range and from any angle and this also extends to the flak shirt and flak pants worn by tunnel rat regiments and potentially what is worn under the armor of all Guardsmen. Flak armor and lasguns are almost uniquely used by the Imperial Guard whereas nearly all rebels and cultists they fight use either mesh armor, armor weave, scrap plate that might eat a stub pistol, or nothing and usually wield stubbers and maybe a few autoguns. Since about eighty percent of the Guard only ever fights rebels and cultists, this makes the average Guardsman practically a Space Marine to his normal enemies which is part of the reason why flak armor and lasgun are standard issue instead of carapace or powered armor. There simply is no point in spending the money to issue better weapons and armor when you're practically invulnerable and invincible to your normal enemies already. When there is something like a WAAAGH!!! or Black Crusade or whatever else, just throw more men at it and lots and lots of tanks and artillery. Since the average Guardsman is nearly invincible against the average rebel or cultist, there are many armored and artillery regiments available to throw at space horrors. It all works out pretty well unless you're one of the ground pounders who has to keep pressure on an anti-tank position while being shredded by anti-infantry you also have to keep busy while your own heavy support gets into position without being killed so they can save your ass so the tanks can come and kill everything. But hey, that's what conscripts and especially Penal legions are for. Literally.
Also, keep in mind that most aliens the Imperium fights are less advanced than the Imperium. Usually, flak armor and lasgun is sufficient even without overwhelming numbers. Basically, ninety percent of the time the Imperial Guard is to the alien threat of the day what the Tau are to the Imperial Guard (but with more tank and less weeaboo) combined with overwhelming numbers. They really only face a challenge when fighting a true Chaos invasion, occasional non-Chaos human renegades and/or pirates, lost human colonies who never met the Imperium before and thought they could take it on, Tyranids, Necrons, Orks, and whatever flavor of Eldar (or a minor xenos faction advanced enough like the Fra'al or Tau, which is very rare). Ultimately, the average Guardsman is no joke. There are literal trillions of them, and this is a low estimate. If only the cheap power armor STC were found. Oh wait, it was but the Ecclesiarchy kept it from the Mechanicus. Thanks guys!
A standard-issue Imperial Flak Vest is rated to stop a lasgun blast outside of close-range. And, assuming we are using relatively high-end calcs of lasguns firing at 19 "megathule" laser pulse, meaning a Flak Vest can stop a 19 megajoule laser dead in its tracks. Assuming "megathule" is a 40kism of "megajoule".
To put that in perspective, one of the most powerful AT guns in WWII was the German 12.8cm Pak 44, which was basically a naval artillery gun on wheels (or tracks). This gun was powerful enough to kill buildings, let alone tanks, and it had so much penetration that it was basically impossible to carry enough armor to stop it. The 12.8cm Pak 44 had a muzzle energy of just under 13 megajoules.
HOWEVER, we need to take careful note that Lasguns have a literal dial-a-yield setting. It is more likely that the average lasgun have its damage output in line with the average autogun (essentially modern 21st century rifles and the literal ballistic counterpart to the Lasgun with higher penetration but lower stopping power), with certain unorthodox Guardsmen increasing their Lasgun firepower to that of Heavy Stubber capability (AKA .50cal) at the expanse of increasing wear and tear deterioration on the Lasgun's optics and pissing off the local Enginseer. After all, there is a reason why Long-Las snipers have to carry multiple spare barrels,
since their Long-Las is fixed permanently in the maximum power settings because long-las fire the entire charge of a hotshot power pack in one blast.
Moreover, lasers have the unfortunate side effect of optical dispersion. At longer range, the photons inside a lasgun would disperse either due interacting with particles in the air, hitting microbial matter or simply lacking zero-mass to retain its focus. In a nutshell, at a certain distance, the light from a lasgun would have dispersed to such a degree that it would feel like a warm summer heat a kilometre away from the original firing point. Think of it this way for better context, a laser pointer from Earth would have a diameter of around 500 meters when aimed at the surface of the Moon. In order to keep the laser focused, it would require a bigger and more powerful generator to keep the photons together from dispersing apart. Ergo, even if the Guard are fighting enemies with lasgun equivalents, if they do not know about light dispersion, normal Flak Armour would be sufficient in protecting the wearer at longer ranges.
With all that said and done, assuming every Lasgun has its fire settings switched to maximum, it is no wonder that the commonality of Lasguns makes it more odd to NOT see the usage of Flak armor in the hands of Imperial soldiers. And that equipment is considered sub-par in 40k. (This was partly paraphrased and partly quoted from user Rofl Tank in a YouTube comment about the Imperial Guard). Keep in mind that if a guardsmen was hit with a '13 megajoule' shot they would likely have every internal organ scrambled and sent flying back as the flak armor kept it from penetrating, but that energy had to still go somewhere. As is often the case when it comes to armor, the man is the point of failure. Assuming, of course, that the energy capture of the armor is terrible, which would be very 40k. But if it performs like modern plates, it should take it with little complaint.
Flak armor comes in incredible variety. Some even have carapace armor sewn into them. There are also types of carapace armor that are technically a variant of flak due to being flak armor with sheathes plates of carapace in varying thickness and number can be inserted or removed. Art Almost always portrays flak armor with cracks and craters in it, making for very weird implications as to what common flak armor actually is. While tabletop has its crunch, lore flak armor is largely unpredictable.
Why It Kinda Sucks
That said, when it comes to protecting its user from guns which shoot 19.05mm APHE rocket bullets, hyper-sonic monomolecular ninja stars, bolts of compressed plasma as hot as a star's core designed to penetrate tank armor, molecular disassembling beams which can break down virtually any material into their constituent atoms, and similar weapons which are the standard equipment of many factions of Warhammer 40000's tabletop game, flak is of little more use than a common T-shirt. Of course, if they miss, and shrapnel is thrown, the armor will probably save you, which is its purpose anyway.
Flak armour is also fairly cost efficient; it offers some protection in combat while costing less to produce, ship, and maintain than the soldier wearing it. This becomes less true as the armour becomes more complex. The quality of flak armor also varies wildly between manufacturers. Flak armor made on Mars, for example, would likely be space magic.
Also, most enemies are going to be suppressed by the large quantity and concentration of heavy weapons used by the Imperial Guard even without considering the platoon support weapons such as heavy stubbers and the squad special weapons. This enables Guardsmen (were they real) to get close to enemy positions and wipe their foes away with close-range lasgun fire. Since lasgun penetration radically increases the closer they get, this would logically be highly effective. So, most Guardsmen need to to be far more concerned with indirect fire than being hit by enemy infantry directly and flak armor protects very well from indirect weapons such as explosives and shrapnel. Remember, there are literally countless Guardsmen but only "millions die every day" in the Imperial Military as a whole. There is a reason for that.
In short, think of Guardsmen as suppressors for tanks against anti-tank threats and spotters for artillery (from man-portable mortars to the biggest of kablooie). Which means more than flak armor would usually be pointless (if you don't care about the lives of individuals that is, and the Imperium neither cares nor can afford to). Soldiers specialized in directly attacking enemy positions wear radically superior armor and weapons; some of the best in the galaxy in fact, even in the Imperium. The average Guardsman, though, is purely supplemental to the vehicles and mostly needed for manning defensive positions in taken territory and telling big guns and missiles where to shoot while gunning down whoever is stupid/insane/scary dogmatic alien mindset enough to rush at them. Much like how modern infantry functions.
The next grade up is Carapace Armor. Which is actually a very large leap in quality. From a reasonable infantry armor sufficient against small arms to an infantry armor capable of tanking self-propelled small cannon shells. There is also an intermediate armor infrequently used called Combat Armor. This is light carapace plates covered in cameleoline and the light carapace is mounted atop flak armor. Said cameleoline makes it incredibly good at camouflage, so a particularly lucky veteran sniper may find themselves in one.
Mannequins wearing abandoned Flak Armor from Space Hulk: Deathwing