Flak Armor

Arguably the most common type of flak armor in the 40k universe, based off the Cadian Shock Troopers standard gear. Noticeably lacking in the wiener department

Flak Armour 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. 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 hits from directed energy weapons comparable in power to a Lasgun.

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 or poor penetrating (will stop a shotgun slug but cracked and broken ribs at most, bruising like Mike Tyson punched you at the least), or equip heavier armour inserts that provide more coverage, or possibly a little less and withstand more powerful rounds, all the way up to armour piercing sniper rounds (assuming the wearer has some sort of steel or ceramic insert, and that also gives 3-10 shots from modern assault rifles.)

With Flak armour gives a chance to not only survive a couple of impacts from from a light machine gun, or several from autoguns, but to remain in a shape 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 Emperorsend 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 is one thing to note: A WH40k Autogun fires bigger rounds than a modern earth Battle Rifle, or LMG. They fire 8.25mm bullets, and this, again, is stopped by Flak. This means the average IG issued gear will take sniper shots with the user surviving. It does beg the question of how big LMGs are in WH40k.

Why It Kinda Sucks

That said, when it comes to protecting its user from guns which shoot .75, or (19.05mm) rocket bullets, hyper-sonic monomolecular ninja stars, bolts of compressed plasma as hot as a star's core designed to penetrate tank armor, and similar, they are 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 it's 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 next grade up is Carapace Armor.