So you've been managing well with your current skills, dispatching enemies with your familiar attacks and weapons. Now you've encountered a Heavily Armored Mook. Using your basic attacks and abilities you ... fail to hurt the Mook at all. Not good.
So what do you need? You need Armor Piercing! Basically, it's an attack, weapon, or munition that rips through the armor that your enemies wear (or their special shield, or whatever's protecting them), often as if they had no armor to begin with. Sometimes this goes to the point of Tactical Rock-Paper-Scissors, with the anti-armor weapon useless against foes with no armor; another option is that the weapon or attack will destroy the armor, after which your normal attacks will work. Destroying some kind of armored shell is a pretty common first step in a multi-stage Boss Battle. This can also be turned against the player as well; for example, if normally their life bar only decreases once their armor bar is depleted, late-game enemies might get attacks that damage both bars at once, or even ignore the armor bar entirely.
Obviously, this isn't quite how things work in real life.
It is true that some weapons tend to be more effective than others against a given type of material. Padded clothing for example, can protect against cuts and blows, but is easily pierced by a sharp object. However, fiction tends to treat armour-piercing properties as being able to defeat any protective measure with ease, while weapons that cannot pierce armour are considerably less effective, to the point of doing little more than Scratch Damage at best.
Firstly, it is possible for a weapon to cause considerable damage without managing to penetrate protection. A Bullet Proof Vest may be able to stop a bullet, but will rarely leave its wearer completely unscathed. This is elementary physics at work: all the kinetic energy that went into propelling the bullet has to end up somewhere, after all. This also applies to tanks as well: if struck hard enough, armour plating can deform, which leads to portions of the inner facing breaking off and turning into dangerous shrapnel.
Secondly, there is no such thing as a weapon capable of "ignoring" armour. Anything designed to breach armour, whether it is a solid projectile or a shaped charge warhead, will encounter some level of resistance. Moreover, the actual effectiveness of a weapon against armour is dependent on factors other than its design and construction. A solid armour-piercing shot from a tank cannon for instance, is strongly affected by environmental conditions (i.e., air resistance and gravitational pull) that will reduce its power over longer distances.
See also Depleted Phlebotinum Shells and Clothing Damage (if the armor is destroyed). Might cross over with Elemental Rock-Paper-Scissors if the armor is made from one element and the skill or weapon uses the opposed element. Not related to Armor-Piercing Slap or Barrier-Busting Blow. May be used in combination with Armor Is Useless and/or One-Hit Polykill. Compare Anti-Armor, when a technique does more damage instead of bypassing defenses.
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In Blood+, Saya's katana cannot cut through James Ironside's carapace. The Schiff use polearms that do the job nicely though.
The Armor Piercing power advantage reduced defending armor by half versus the attack.
The Penetrating power advantage: even if the attack is completely stopped by enemy defenses, a little (averages to 1/6) will always get through.
Most modern day RPG's have some kind of armor piercing bullets.
Shadowrun had APDS (Armor Piercing Discarding Sabot) bullets for firearms.
GURPS has various "armor divisors" from 2 through infinity. For example: in the Ultra-Tech book even handgun bullets can be loaded with the High Explosive Multi-Purpose (HEMP) warhead which reduces the effectiveness of armor to one fifth normal by using a sort of shaped charge.
Most weapons in Warhammer 40,000 have an armor piercing stat that specifies how good an armor they can bypass (in game terms, it deprives the target of its armor-based saving throw). Previously, special melee weapons called Power Weapons did not allow armour saves, ever; as of 6th edition they have Armor Piercing stats just like the guns do, albeit very high ones.
Given that most infantry weapons have an AP stat of at least 5, this renders any models with an armour save of 5 or 6 pretty much unprotected against enemy fire. Like the Imperial Guard, who are unfortunate enough to be squishy humans. And armed with a laser gun with no armour piercing value whatsoever.
Tau rail weapons are terrifyingly good at this. One piece of fluff describes a tank that had been taken out by one, a hole in one side matching the hole in the other. The inside was still red and sticky due to the crew having been liquified by the hypervelocity round's passage. Indeed, in the game they have the highest Armor Piercing stat possible, which incidentally causes a damage bonus against vehicles if the player rolls a "penetrating hit".
Its fantasy counterpart has Armor Piercing Attack as a special ability of certain weapons, like guns and crossbows. It lowers the target's armor even further (in addition to the modifier based on the strength of the weapon used).
In the Warhammer 40,000 roleplaying games, weapons have a penetration value, which determines how many points of armor they ignore on a successful hit. The rules are much more favorable to the defender than the tabletop war-gaming rules, since no weapons ignore armor altogether, although some weapons like the MP Lascannon and the Exitus rifle have such high penetration scores that only the strongest armors will be of any use against them.
The Hero Clix version of the feat card allows any character who wields it to still do 1 damage even if it would normally be reduced to 0. They still have to have at least a damage value of 1, though.
A clip of ammunition with this in Mekton Zeta costs four times as much as a normal round and halves the target's armour value. (For a similar price, you can skip attacking the target's mech at all and shoot electric bullets, which bypass armour and directly affect the enemy pilot.)
BattleTech offers armor-piercing shells for its standard and light autocannons, which may inflict critical hits — though no internal structure damage as such — even through armor. (This is in addition to the usual chance of a lucky roll on the hit location table.) Many players don't consider that effect worth putting up with their reduced accuracy and halved number of shots per ton, though.
Earthdawn had armor-defeating hits: basically, if you rolled well enough on your attack role, you could ignore the effect of your target's armor. In addition, some weapons and spells (razor orb being the most notable) were designed to cut through armor so a lower roll could still be an armor defeating hit.
In Magic: The Gathering, spells ending in -dict (such as Diabolic Edict), and the annihilator ability, require your opponent sacrifice a card, getting around protection, absorb, indestructibility, damage prevention, high toughness, regeneration, and everything else. Unfortunately, your opponent still gets to choose which card he sacrifices.
In Dungeons & Dragons 3ed/3.5, a number of attacks can ignore certain categories of defenses, and become a very major part of the strategy in advanced play. Many spells ignore armor; a handful also ignore spell resistance and saves or deal damage of a type that is virtually impossible to resist - spells with all four of these traits are highly valued. A rogue or ninja's backstab attacks, conversely, ignore dodging and a number of related defenses which are often the best options against magic damage. Lances do not actually negate any defenses outright, but due to the way accuracy and damage reduction function in the game possess characteristics that make them extremely effective against almost all forms of defense.
Certain monk builds based on grappling also negate armor.
In 3.0, a combination of very high critical hit rates and the buff Bless Weapon were able to negate armor on 60% or more of all attacks. Nerfed in 3.5, where critical buffs cannot stack and are exclusive with any buff that would actually make them worthwhile anyway.
In the 4th Edition, attacks that use a weapon but target Reflex are effectively armor-piercing attacks. Armor-Piercing Strike and Piercing Strike are attack powers that are Exactly What It Says on the Tin.
In Yu-Gi-Oh!, when a monster in Defense Position is attacked by a monster whose ATK is higher than its DEF, the Defense Position monster is destroyed, but the controller of the destroyed monster takes no Battle Damage. However, several Effect Monsters, such as this one, are designed to not only destroy Defense Position monsters, but also inflict the difference between their ATK and the DEF of the defending monster as Battle Damage to the controller of the defending monster.
In Big Eyes, Small Mouth, weapons can have "penetrating" or "piercing" properties to bypass some of the damage reduction from armor or force fields. Conversely, the "non-penetrating" defect increases the effective damage reduction in exchange for decreasing the point cost of the weapon.
Ancient Domains of Mystery: There is a weapon called phase dagger that completely ignores the armour of the target. The downside is that on each hit, the game throws at you a message about you easily cutting through the armour, which requires you to press [more] much more often. And the dagger itself doesn't do much damage anyway, making it useful only against Heavily Armored Mooks.
Final Fantasy X does this with armored enemies, who require a specific weapon type to get past it. Fortunately, they begin to show up just as you get the biggest Badass around, whose weapons all come with the armor-piercing ability, into your party.
The "Armor Break" skill will remove the "armored" enhancement from its target, allowing you to damage it normally with non-piercing weapons.
In all the Final Fantasy games, Flare and Ultima tend to be armor-piercing attacks.
In the DS version of Final Fantasy IV, an Augment you can receive after defeating Bahamut is this for the magic-reflecting spell Reflect.
Final Fantasy XII had guns and measures, both of which only calculated the weapon's strength for damage. So while they were good against enemies that had high defenses, it didn't matter how strong a character was when using them. However, it doesn't help that some of the tougher enemies only take a 16th of the damage from the two weapons.
Final Fantasy VI tends to allow its strongest attacks (Bum Rush, Drill/Chainsaw, Ultima) to ignore defence, and they usually always hit too. Unfortunately, this effectively neuters the design philosophy behind some enemies, usually of the "evasive and tough, but with extremely low health" variety. The most extreme example would be the Cactrot, which follows that model exactly: it has a perfect defence rating (all physicals do 1 damage) and is extremely hard to hit with physicals and magic alike. Catch? It has 3 HP, so anything that always hits and ignores defence, no matter how weak, will kill it.
In The Legend of Dragoon, the group knew they'd stand no chance against the Divine Dragon on normal terms, so they went on a quest to find the Dragon-Block Staff (made specifically for fighting dragons), thus allowing them to fight a significantly de-powered Divine Dragon with a chance of victory (which is funny, because it still manages to be That One Boss).
Mass Effect 2 has three different kinds of non-health defenses. Powers and weapons receive damage multipliers against specific defenses. Powers will not perform their full effect against armoured targets (Attempting to knock down a target with a concussive shot while they are protected will just stagger them), but may still damage their protective barrier (and may even be unable to damage targets who are unprotected - Overload, for example, is a counter to shields and synthetic enemies, and does no health damage to organic enemies, only being able to damage their shields)
Mass Effect 3 buffs armor piercing attacks, making them work against improper defenses at 1/3rd effectiveness, or their listed damage. Overload is treated in a particularly nice, given that it stuns humanoid targets, and can be upgraded to knock unprotected targets off their feet entirely for an even longer stun time. Although, this fits in with the games overall greater focus on powers.
Certain guns and mods also give your bullets penetration, which doesn't stop armor, but lets it go through cover, which includes the other-wise invincible riot shields. A Javelin sniper rifle with maximised penetration mod, maximised high-velocity barrel, and Garrus's specialty ammo can go through thicker cover than actually exists in the game.
Variant: acid attacks in Mass Effect 1 bypass your shields. The effects of this range from "mildly inconvenient" if you have armour with plenty of damage reduction and Immunity up, to "all right squad, we're taking a break to weep" when a thresher maw begins to spit holes in the Mako or shield-focused Tali gets mugged by a swarm of rachni workers.
In Dragon Quest IX, the Metal Slime family is almost completely immune to physical attacks and totally immune to magical attacks. All critical attacks, bypass physical defense, rendering them the only practical way to take down the stronger Metal Slime variants.
In Aliens Versus Predator 2, Praetorian Xenomorphs are immune to most standard gunfire, and only vulnerable to heavy weapons or special armor-piercing bullets from the lighter ones.
Of course, Predator players have to take down Praetorians the manly way: with their armor-piercing spears.
One of the Samurai's abilities in Disgaea 3: Absence of Justice allows half of his basic attack's damage to ignore the enemy's defense. There's also a gun skill called Armor Piercing, but it subverts it by not ignoring the opponent's defense in any way.
FEAR 2 has several weapons which disregard or degrade the functionality of armor, both for enemies and the player.
To be more specific: all weapons, especially in the first game, deal part of the damage directly to the health. However, in most weapons the damage that isn't absorbed by the armor is a very small fraction of the total. However, there's a weapon in each game (the Penetrator in the first game, the Hammerheadin the second) that ignores armor almost completely, making them true examples of this trope.
The Wing Commander series, starting with Wing Commander 2, features capital ships with shields or armor that is too heavy to be threatened by the weapons normally carried by fighters and smaller warships. To overcome this, heavy fighters and bombers can carry large, bulky torpedoes which are designed to "phase" through the enemy's shields and strike with a nuclear or antimatter warhead. The process of finding the right frequency for the shields justifies you needing to make long approaches towards an enemy ship which is actively trying to kill you before you can finally release the torpedo (and hope it doesn't get shot down itself). Later in the series, it goes back and forth between whether non-torpedo fighter weapons can or can't harm capital warships.
Warcraft 3 has various damage and armor types. Certain damage types may deal more or less damage depending on armor type, while armour amount also influences how much damage is received through percentage reduction. One type of armor (Divine) is practically immune to any damage except one type of damage (Chaos). Spells tend to ignore physical armor when inflicting damage, but may instead be affected by some abilities or fail to work altogether on some unit classifications.
In World of Warcraft, this trope used to exist as Armor Penetration, a very desirable stat for classes that mainly inflict physical damage, such as rogues and warriors, but it was removed due to balance issues, though those two retain abilities to reduce the opponent's armor with a debuff for the benefit of their allies.
Speaking of Blizzard games, both Starcraft and Starcraft II also have two different armor/damage systems in play. In both games, the amount of armor on a unit subtracts from the damage dealt by an attacking unit: if an attacker deals 3 damage and the target has 2 armor, 2 damage will be subtracted from the attack and only 1 damage will actually be dealt. In gameplay terms, this means that weaker attacks will be disproportionately less effective than higher damage attacks against armor, although this could be easily overcome by Spam Attack from faster attacks or through sheer numbers.
As for the secondary armor/damage systems, Starcraft has one in which different attack types may do half, three-quarters, or full damage depending on unit type, with "concussion" type attacks dealing full damage to "light" units, three-quarters damage to "medium" units and half damage to "heavy" units. Starcraft II does away with the system, opting instead for one in which some units receive a bonus to attack against certain unit types. A Terran Marauder for example, deals 10 damage to most targets, but receives an extra 10 damage against any "Armored" type ground unit for a total of 20 damage. Like Warcraft III, abilities in both games ignore physical armor but may be affected by unit type, with Irradiate capable of dealing up to 200 damage on biological units but remaining completely ineffective against mechanical ones.
In Fallout 3 the Deathclaw Gauntlet ignores armor entirely, as do the Deathclaws' own attacks. The Tri-Beam Laser Rifle in Broken Steel also has this effect when wielded by Super Mutant Overlords, one of a quartet of Demonic Spiders introduced in this DLC. No such luck for the player. The Tribals and Swampfolk in Point Lookout have a similar hax effect with their weapons, the double barreled shotguns being the most deadly, since the armor-piercing damage is applied to each individual pellet, possibly resulting in a One-Hit Kill if all of them connect with the player. Likewise for the Feral Ghoul Reavers' Deadly Lunge, and their gore grenades, which also instantly cripple body parts on a direct hit.
Largely averted in Fallout 2 but only due to bad design. Hollow-point and other "regular" ammo is generally superior to armor-piercing ammo in any given situation with the AP Needler ammo and the AP rocket (!) ammo being exceptions.
Played straight in Fallout: New Vegas. Due to the way armour is implemented in the game, players may need to use special weapons, ammunition and methods of attack to penetrate an enemy's armour and cause meaningful damage. AP rounds for guns, overcharged energy cells for energy weapons, melee and unarmed weapons with the "Ignore DT/DR" feature are the most obvious, but using a weapon that deals a large amount of damage per shot/swing (such as the Anti-Material Rifle or the Super Sledge) will also do the job. And of course, if you're the sneaky type, you can also go for a sneak attack critical, which has its damage added to the original attack and thus isn't nearly as affected by DT.
On the enemy side, there's also Giant Rats/Rodents of Unusual Size. If normal-sized rats can chew through so many things, imagine what giant rats will do to humans.
The Proton Axe from Old World Blues does 20 bonus damage points against robots and Power Armor, effectively cancelling most of the armor's DT, which includes the player when Lobotomites are wielding the weapon.
Lonesome Road's resident Demonic Spiders, the Tunnelers, also seem to bypass the player's damage threshold. At high levels, they deal as much damage as the main game's Deathclaws.
Featured in Paper Mario with Watt, whose basic attack ignores the enemy's defense (making her one of the most useful partners in the game, especially against high-defense enemies), as well as badges which would apply this property to your Jump or Hammer attacks. (Items and the Star Storm power also ignore defense.)
The Thousand-Year Door also features many of the same badges (and items/damage-dealing Star Powers), and included Yoshi, whose spit attack had this property (and was needed to beat a mid-level boss who pretty much had infinite defense), as well as the secret character Ms. Mowz, whose attack is literally an Armor-Piercing Slap. However, there are also several enemies (mostly Elite Mooks) in this game that can pierce your defense, negating all defense badges you have equipped and leaving you with the standard choice of a regular Guard to stop 1 damage or a trickier Superguard to stop all of it.
In Master of Magic for units with Armor Piercing attack a target has effectively 1/2 (rounded down) of its normal defense value.
Same in the Dominions series (though the defense stat is called "protection", while the stat called "defense" is really evasion). Dominions also adds armor negating, which completely ignores the target's armor. (No, it does not make the armor negative!)
In Master of Orion 2 some weapons can be made in armor-piercing variants (and some in shield-piercing, for that matter) so that damage that passes through armor/shields is applied to the target ship's internals immediately.
Heavy and Xentronium armor negate armor bypassing, but neither can protect against an attacker equipped with an Achilles Targeting Unit device. Shield piercing weapons can be stopped with the Hard Shields device.
The iOS port called Starbase Orion has the Gauss Turret, which completely ignores armor. The downside is that it doesn't do much damage to shields and requires the ship to close in to short range for maximum effectiveness.
In Impossible Creatures, creatures that attack with quills or horns will bypass a portion of the enemy's defense.
Iji features this due to the fact that everything is made of Nanotechnology. To wit, small arms fire such as the machine gun or pulse rounds will chip away at your armour (which is really more of a forcefield) until it is depleted at which point a Hit Point is sacrificed to replenish it. However anything which hits you (as in, a rocket, BFG round or a punch) will deal actual damage to your body directly (and send you flying across the room to boot).
Starting in Space Empires IV, weapons can now skip armor, in addition to shields.
Sword of the Stars II goes for Sequel Escalation on the original. Instead of Critical Existence Failure, ships now have "armor matrices" that need to be brought down until you have a clear "path" through to the internal structures, with any shot that penetrates the armor dealing critical damage, such as damaging key systems and killing crew. Also, "armor layers" enable a ship to No Sell any attack that doesn't have a high enough AP level. Every weapon has its own "damage pattern" against the armor matrix; for example, some like lasers have better armor penetration than mass drivers. However, that means that each subsequent laser shot has to hit the same exact column to make a difference, while mass drivers can wear down a larger area of armor with each shot. Rapier vs mace in other words. More advanced weapons create dispersed, deeper effects making such painstaking accuracy no longer necessary. As in Prime, Mesonic Torpedoes are the best at penetration, going through 10 permanent armor layers. This breaks the rules of the setting, since the normal maximum of permanent layers a ship can have is 5.
The Terran EMP Shockwave in StarCraft destroys the shields of anything it hits. Not very useful against the Terran or Zerg who don't have shields at all, but a Protoss Archon has 350 shields and 10 health...
Borderlands: one of Mordecai's skills enables him to bypass enemy shields. Against the Guardians, their health bars being tiny, it is absolutely devastating. In the sequel, one of his quest rewards is a sniper rifle that has this very effect.
A bugged legendary weapon, the Vladof Vengance, has this effect. In theory, it should allow the machine pistol to occasionally fire rounds that pass through armor to damage health. In practice, this effect can also get passed on to any of the other weapons in your inventory at the time. Shield bypass ability on something with enough damage or enough dakka (or both!) can veer straight into Game Breaker territory.
UFO Aftermath has no less that seven damage types: soft, universal, hard, laser, plasma, burn (fire and acid), paralyze and warp (the more armor you have, the more it hurts). Transgenants and armors have separate resistances for each. There is an eighth type referred to as "exception" for a few weapons like the psionic crusher.
Company of Heroes gives AP bullets and shells as special abilities for American and British machine guns and AT guns. The former ability turns the dedicated anti-infantry HMG into a feared light-vehicle counter, while the latter boosts the penetration of the AT guns. As the British 17pdr is already one of the best antiarmor weapons in the game, the AP-Discarding Sabot ability makes it one of the few weapons that can counter King Tigers.
While traditionally in Armored Core series, Attack Power is the only attribute to watch out for weapons (meaning, how much you need to pump it to any given target to make them go kablooey), 4/for Answer introduced the Force Field-like Primal Armor and consequently, "PA Penetration". Energy weapons, sniper rifles and railguns do this job really well, and since about a good 4/5ths of the game's armor isn't exactly energy-weapon proof, this can only go wrong, were it not for the balancer mechanics.
Ragnarok Online not only has weapons that ignore defense, but a dagger called the Ice Pick, which isn't reduced by the target's defense and then proceeds to do more damage depending on how much defense it had. Wearing armor actually makes you take more damage. Thankfully, they are quite rare.
In Guild Wars, a few skills have an "armor penetrating ability" reducing a target's armor by a percent against the one attack. Due to how armor works (Extra armor is multiplicative in how much damage it reduces), this has a varying effect depending on the amount of armor. Guild Wars weapon skills that add damage also add a fixed amount of damage, that is unaffected by armor, making them useful against highly armored targets.
Pokémon has Brick Break, which breaks any Reflect and Light Screen shields the opponent has set up before damaging the opponent, as well as moves like Chip Away and Secret Sword, which do damage while disregarding opponent's stat boosts in defenses.
The Abilities Unaware and Infiltrator ignore stat changes and Reflect/Light Screen (as well as Substitutes in Gen VI) respectively.
Sound moves such as Hyper Voice can bypass Substitutes in Gen VI.
In the Total War series certain units (such as longbowmen or those armed with axes in Medieval I and II) actually put heavily armored units, including those with the extremely expensive armory upgrades, at a disadvantage with this ability.
Post-Marian Reforms legionaries in Rome: Total War can shrug off almost any kind of missile attack due to their heavy armour, but a volley of armour-piercing javelins from the right angle can devastate them. Especially fun with Bull Warriors, who have very poor defence for their price but sport the most pimped-out AP missile attack stats of any unit in the game.
In Soldier of Fortune, high-power weapons such as the sniper rifle can pierce both the player's and enemy's armor.
The Pierce skill in Shin Megami Tensei III: Nocturne, which allows those who have it to ignore the enemies' physical resistances, immunities, and drains (but not repels). You get it just in time, too, since that's the point in the game when your randomly encountered enemies may have as many as four or five out of eleven elements be useless against them. Having the skill, and jumping through a few hoops to be able to get it on your Mons, changes the last part of the game from insanely hard to merely sanely hard.
Devil Survivor and its sequel have it too, but only work for physical attacks. Still, being able to pummel bosses who otherwise drain or nullify physical attacks with a dose of Deathbound or Multi-Strike is quite satisfying.
Shin Megami Tensei IV expands this to six different Pierce skills: one for normal Physical attacks, one for Gun, and four elemental Pierces. All are exclusive to the DLC, the Elemental Pierces all learned by Aeshma, Phys Pierce by Sanat and Gun Pierce by Masakado's Shadow form. Each of the Four Archangels learns an elemental attack that will have the Pierce feature built in: Tornado of God, Hailstorm of God, Lightning of God and Inferno of God.
Perfect Dark has the Callisto NTG's Secondary Fire setting (High-Impact Shells) and the Farsight sniper rifle. Both will make short work of any shield.
Turned against the player in Killing Floor - the Siren's scream ignores armor entirely, while attacks from the Fleshpound and Patriarch typically damage both health and armor.
Deus Ex: Human Revolution has a realistic take on this: AP mods for your weapons are most useful if the enemy has heavy armour, with the upgraded pistol ignoring armor entirely, but don't do much better than normal rounds against lightly-armoured enemies due to overpenetration.
X-COM: UFO Defense and Terror from the Deep starts you with armor piercing weapons. Subverted, since that's only the damage type rather than the ability, and the non-armor piercing attacks are better at breaking through armor. To be more specific, the armor piercing weapons won't overcome the high armor and damage reduction of Lobstermen and Triscenes.
In Mega Man Battle Network there are four kinds of armor that can be pierced - Shielding (which yields to Break), Shadow (which yields to Sword), Barriers and Auras (which yield to Wind), and Invisible (which yields to Cursor).
Pistol and machine guns in the BioShock series both have variant armor piercing rounds, as well as standard slugs and anti-personnel rounds. Everything was more vulnerable to one and less vulnerable to the other, with the armor piercing being useful against machinery and Big Daddies, while anti-personnel pistol rounds would one-shot most splicers.
Battlefield 2 had an unlockable anti-materiel sniper rifle for the sniper class, the M95. It's main claim to infamy among the player base was its ability to snipe the pilots of helicopters through their armored canopy glass, something no other gun would do.
Command & ConquerGenerals and Zero:Hour subvert it by giving the GLA upgrades for armor-piercing bullets and rockets, but those merely increase damage dealt by units using them by 25%.
In King Arthur The Roleplaying Wargame, certain units can get the armour piercing trait, which reduces enemy armour by 50%. Oddly enough no archer units can get it, but then again King Arthur's archer units are generally considered Game Breakers already and probably don't need any more help.
Arcanum has two guns, both BFGs, that do a sort of this. Blade Launcher ignores Armor Class, allowing you to hit more often (and is the second most powerful gun), while Riffled Cannon ignores Damage Resistance, allowing you to do full damage each time you hit. They both, however, consume 6 bullets per shot.
This type of attack is nearly mandatory in World of Tanks - not every gun has good HE damage, meaning that often times your ability to deal damage relies entirely on penetrating the enemy's armor from any direction possible.
Justified since the game is based as closely as possible on Real Life tank warfare, where armor penetration is king.
In Batman: Arkham City, mooks start wearing protective armor and donning SWAT riot shields to combat Batman. To counter, Batman has two attacks that get through this - the Beat Down attack, which has Batman rapid fire punches before delivering a knock out blow, and an attack that has Bats climb their shield, then pounce on them.
Breath of Fire II had the spell Chop Chop which did a flat damage of 25 to any enemy. This made it a one hit kill against enemies that had high enough defense to reduce all attack to 1 or 2 damage since even the toughest of these only had 20 health. Too bad it was easily Lost Forever if you didn't go after it as the first thing you did when arriving at the eastern continent.
Due to the combat mechanic of Galactic Civilizations II, armor is only really effective against mass drivers. Lasers and missiles are affected by armor, but only by the square root of the armor rating (e.g. an armor rating of 9 will only be effective as 3 against non-mass drivers). The same goes for shields (counters to lasers) and ECM/point-defense (counters to missiles).
Team Fortress 2: Sort of; in Mann vs Machine, Giant Robots are immune to the Spy's Back Stab due to their stronger armor, thus requiring the Spy to upgrade his knife to inflict a certain fixed damage upon the Giant Robot on back stabs (each of the four upgrades increases damage by 25% of the maximum 750).
Elona: Different types of weapons ignore different (small) percentages of armour when calculating damage. For instance, a club would ignore 0% of armour when attacking, while a long sword would ignore 10% of armour. Lightsabers ignore 100% of armour, and there's equipment which can be worn which will occasionally make any blow you land ignore 100% of armour, regardless of what weapon you're using.
In the Fire Emblem series, the recurring skillnote or tome in the Game Boy Advance games Luna either halves or negates the target's Defense or Resistance (whichever is applicable to the attack).
The series also tends to have a few weapons that deal extra damage to heavily-armored units, such as Armorslayers, Heavy Spears, and Hammers.
Mega Man X: Command Mission has several weapons that your party members can equip that have armor- or shield-ignoring properties. The base damage on them is rather low, however, but they're quite helpful against enemies with high defenses, and also bypass defense-enhancing buffs. Some enemies and bosses also have at least one piercing attack in their arsenal, subject to the same properties but sometimes a lot more dangerous due to the foe's high attack power.
Certain guns in the Syphon Filter series ignore or destroy armor, for example, the snipers in The Omega Strain completely nullify your Armor gauge on the first shot, with the next likely being a One-Hit Kill. Starting with the second game, some enemies fire headshots that instantly kill regardless of armor condition.
Jagged Alliance 2 has armour-piercing ammunition for most weapons; in fact for some weapon classes it's actually the default type. Its effectiveness varies somewhat, as it only gives an attack bonus versus armour and -realistically- a penalty versus targets who aren't wearing any. Since almost every single enemy Mook has some kind of body armour, even a Vietnam-era flak jacket and a steel helmet, it's hollowpoint ammunition and its damage bonus versus unarmoured targets that ends up being highly situational instead.
Dwarf Fortress doesn't have a dedicated armour-piercing weapon class as such, but bludgeoning weapons suffer only a minimal penalty versus plate or chain mail, whereas leather armour is more effective at reducing blunt-force trauma but offers limited protection against stabbing or cutting weapons.
The Engineering boff power "Directed Energy Modulation" allows your energy weapons to do a percentage of their damage directly to the target's hull instead of having to batter down the shields first.
Plasma procs cause a damage-over-time effect that eats hull.
Transphasic torpedoes do half-damage direct to the hull when they hit a shielded target. The Voth love to fire transphasic chroniton torpedoes in salvoes, and they hurt bad if you're in a typical Glass Cannon-built tacscort.
Gotcha Force features two different ones - drills on the Drill Robot, and the energy lances of the Atlas Tank, the Proto Atlas, and the Atlas Robot (which can switch between the previous two forms). They can go through obstacles, other enemies, and even the rare barriers that some borgs can generate. As one of the borgs that can make barriers is the Final Boss, they're the go-to choice for the final assault.
There are several characters in Worm who can produce this effect:
Immediately before the Leviathan attack, Armsmaster invented a modification of his Halberd that would allow it to cut through just about anything by severing the molecular bonds.
When Shadow Stalker is in her shadow form and fires her crossbows, the bolts remain in the shadow form for a little while before returning to an interacting-with-matter-normally form.
Flechette can imbue her weapons and ammunition with her power to make them unaffected by normal matter for however long she wishes — meaning she can time their return to coincide with the moment they intersect with her target. The end result is that her attacks penetrate virtually any defense, even Leviathan, who was too tough for Armsmaster's aforementioned Halberd to fully penetrate.