Action Hero Bob is sneaking into the villain's secret base when he's suddenly spotted! As the enemies open fire with their automatic weapons, Bob grabs the nearest mook and lets him take the shots instead. While the hapless henchman soaks up the bullets and twitches like a spastic marionette, Bob drags him along and heads for safety.
Fictional works love this trope, giving the hapless Mook the stopping power of twelve inches of reinforced concrete. It's popular with anti-heroes, as it serves to show off the hero's cold-blooded resourcefulness. It's also a trope used by a Jerk AssTechnical Pacifist; they aren't supposed to kill people, so they grab the nearest enemy and let their opponents take out a few for them. "I didn't kill anybody," says the blood-covered pacifist, after dragging numerous enemies into the paths of other people's bullets.
In Real Life, a Human Shield would probably work against small calibre weapons like handguns. On the other hand, army engineer manuals state that it takes 60 cm (2 feet) of soft wood to stop an M-16 and 120 cm (4 feet) to stop an M-60note The composition of the projectile is also a factor. Unjacketed, high-energy slugs tend to break into fragments and lose a significant fraction of their energy as they rip through living tissue. Lower energy rounds often stay intact, and diffuse less energy, allowing them to penetrate more deeply. But that wouldn't look very cool without some big guns blazing, would it? This trope almost always involves at least one automatic weapon, and the more there are, the cooler it looks. So in Real Life, Bob is likely to find himself in serious trouble; while some shots might stop inside his co-opted mook, the automatics being fired would have enough power for others to pass through and into Bob. And considering the number of bullets usually taken by the mook, some would miss him and hit parts of Bob that are unprotected because Bob is now standing in one place, and his Mook is unlikely to be bigger than Bob to cover him completely.
The trope can be justified somewhat if the Mook is wearing protective gear like a Bullet Proof Vest - the vest stops or slows the bullets on entry, they're slowed more by the body and stopped by the back of the vest. Unfortunately this only works for the areas covered by a vest; furthermore, a Mook wearing full body armour would probably survive as well, and won't appreciate being used in this fashion. It might also be somewhat justifiable if a mook is only being used to block one or two bullets instead of the usual fusillade of fire, though this would still entail an element of chance.
A Sister Trope to Human Shield, but while that trope talks about using bystanders as psychological protection, this trope covers the bullet-stopping tendencies of the bystander. Related to Concealment Equals Cover, with the mook serving as the eponymous Concealment.
If this results from a video game making units immune to fire from their own team, think of it as a Good Bad Bug.
Contrast with One-Hit Polykill, where the bullets do go through the Mook to hit Bob, Grievous Harm with a Body, for when the mook is used as a weapon, Shoot the Hostage and Shoot The Hostage Taker. Compare Taking the Bullet. Also see Annoying Arrows, Concealment Equals Cover, Improbable Cover, Magic Bullets and Pocket Protector.
Subverted in that she's perfectly capable of catching/deflecting bullets, and only uses a shield because she's Ax-Crazy and believes it will hurt Kurama, whom she despises.
Double subverted since the bullets could be high-powered ones that the Diclonius in general have some trouble deflecting. Plus there's just so many bullets.
Although it's not really a 'human shield' scenario, Samurai 7 subverts viewer expectations about the ability of the human body to stop bullets at close range when Katsuhiro in one of his berserk periods near the end, having lost his sword grabs a machine gun from a fallen mook and fires repeatedly at point-blank range into an enemy. Kyuzo is on the other side of this enemy, and becomes very dead. This is why you should not be careless with guns.
Averted in Hellsing. The whole reason Seras becomes a vampire is an Emergency Transformation because Alucard shoots through her in order to kill the vampire priest using her as a shield.
Luffy himself isn't above using mooks or Buggy The Clown to shield himself from attacks.
When Luffy uses Buggy to shield himself from Mihawk, he wasn't trying to block Mihawk's attacks. He was using Buggy to break Mihawk's line of sight so Mihawk can't see where he's dodging. Mihawk's attacks still go through Buggy like butter and Luffy still needs to dodge it.
He also had no problem using one of Arlong's mooks to shield himself from Arlong's attack. Suffice it to say, Arlong was not happy.
Hody Jones does this to a fellow fishman, what contrasts his personality to Arlong's.
Done in the opening of Gundam 0080 with humongous mecha, one of the cyclops quickly grabs a GM so that another GM shoots him then hesitates. The Cyclops then shoots him through the GM.
Exaggerated in Fullmetal Alchemist. During the Ishballan War, Kimblee uses one of his own Mooks to shield him from an explosion, although the explosion killed the solider instantly, the only thing Kimblee complained about was that his uniform was messed up.
In Ghost in the Shell, Batou casually stops high velocity, very penetrative shots from an SMG using a silent, nameless civilian appearing onscreen for about one second, which is the only screen time of the civilian during the entire franchise. The shots could be interchangeable with shots that passed through a police car door. By the way, the protagonists are all cops.
In an episode of Ghost in the Shell: Stand Alone Complex, members of an anti-cybernetics terrorist/revolutionary group scale ropes during an attempt to stop Section 9's assault on their base. The first group are easily gunned down in large numbers trying this and hang limp on the ropes. The second group to scale the ropes simply pushes the corpses of their allies in front of them to act as shields, drawing surprised reactions from the Section 9 team. A comment suggests this may have been the terrorists' plan all along to level the playing field against the small number of heavily armed Section 9 agents.
Code Geass: Apparently, Cornelia has no qualms about using one of her own subordinates as a Knightmare-flavoured shield should the need arise.
In Naruto, Danzo once used the corpse of a slain assassin attacking him to block spears thrown by several others.
Gantz as the picture above demonstrates. A barrage of full auto can be easily stopped by holding a random corpse in front of you.
Noir features this VERY often. Kirika is probably the one who pulls it off the most, but Mireille and Chloe have used meat shields as well.
Mina uses this against two human attackers in the very first volume.
In one comic Metallo shoots Superman with a kryptonite bullet (the one kind of speeding bullet he is not faster than) and knocks Batman out with a blow to the head and buries the two alive. Batman comes to and blows himself and Superman out of the ground with his utility belt plastique, using Superman as a literal bulletproof human (er, alien) shield (along with the fact his batsuit is fully armored and fireproof).
Used utterly ridiculously in one Batman story, where a character uses a crook as a shield against another, who appears to be shooting an assault rifle at him from pointblank range. What the hell was that guy made of?
A single-issue Punisher story released the same time as The Movie featured a shoot-out in a morgue, with Frank giving the rather sage advice of "Don't hide behind the thin guy."
Inverted in an issue of X-Men where the immortal villain Cameron Hodge was holding Psylocke hostage and dared Cable to shoot through her to hit him. Cable surrendered instead.
"Drat and bother! I sooo wanted to see the look on your face when she died and I didn't!"
The obscure Howard Chaykin comic Power and Glory featured a government agent and a Superman expy with severe neurosis. The expy was a Corporate-Sponsored Superhero that looked good for the public while the agent was the real Badass Normal that was secretly taking down the bad guys. One image from that series involved the agent using the superhero in this fashion as seen on this cover◊.
In Total Recall (1990), when Arnold Schwarzenegger's character is attacked by goons while on an escalator, an innocent bystander catches a bullet while next to him. Ah-nold grabs the man and uses him as a shield. He even spins the dead man around to block more gunfire from the other direction.
In Thunderball, James Bond is dancing with villainess Fiona Volpe when one of her henchmen attempts to shoot him in the back. Bond spins her around at the exact right time that the bullet kills her instead.
Bond also does it in The Spy Who Loved Me, quite disturbingly as the woman may have been meant to be working with the bad guys, but it isn't made clear.
Subverted in Tomorrow Never Dies. Bond kills a guard, then dangles his lifeless body out of a doorway. Sure enough, the henchmen mistake the dead man for Bond and empty their guns into him. However, Bond wasn't hiding directly behind the man, and in this case, he WANTED the villains to think he was dead.
Spoofed in Austin Powers: The Spy Who Shagged Me. Austin uses Robin Swallows (one of Dr. Evil's agents) as a shield against a thrown knife, submachine gun fire, a bazooka round, and breaking his fall from a five-story building. Justified in that she's actually a robot assassin. Watch it here, starting at 8:00.
In a deleted scene, Austin and Felicity are driving around when Dr. Evil's hitmen drive up and spray their car with bullets - but Austin grabs Robin's body out of the trunk to block the salvo.
Happens in the martial arts action movie, The One, with the Big Bad using a Red Shirt cop as a shield from incoming bullets. It actually shows a close-up of the bullets clearly failing to penetrate said shield's bulletproof vest.
Subverted in The Shadow. At the beginning of the movie, a rival opium producer, desperate to escape a confrontation with Ying Ko, grabs Ying Ko's accountant and uses him as a human shield, noting that Ying Ko's men "aren't marksman enough to shoot around him". It doesn't work, since Ying Ko orders his men to shoot through the accountant, which kills him as well as the opium producer.
At the end of the post-bank robbery running gun battle in Heat, Michael Cherito (Tom Sizemore) picks up a small girl to use as a human shield. Didn't do him any good for protection because Vincent Hanna (Al Pacino) was already behind him and drawing a bead.
Happens in Sherlock Holmes during the first fight scene, when Sherlock spots a mook coming towards him with a revolver and uses some fancy martial arts technique to maneuver the mook he is currently fighting into taking the bullet for him.
In The Adventures of Pluto Nash, the protagonist wanders into a trap, at which point he grabs a mook and tries to use him as a bulletproof shield and a deterrent. The bad guys simply choose to shoot both of them. Despite their futuristic guns, they still fail to kill Nash. Possibly, because everyone seems to wear thin Kevlar-like undershirts that protect against any weapon.
V for Vendetta: V uses this tactic to outmaneuver the constables in Jordan Tower.
In Crank, Chev Chelios uses his friend Kaylon's dead body as a shield.
During the Omaha Beach scene in Saving Private Ryan, medics use the body of a dead soldier to successfully shield a wounded man from machine gun fire. The wounded soldier's helmet, however, is completely ineffective.
In the film version of Sin City, a federal agent is used as a shield while Sociopathic Hero Marv hacks up his teammates with a wood ax. This actually seems to be unintentional on Marv's part since he was busy killing one of the agents when the human shield basically ran into the path of the bullets.
Done literally in MacGruber to escape some thugs who are about to gun them down MacGruber uses Piper as a human shield, and they both survive as Piper was wearing bullet proof vest, but MacGruber didn't knew that.
Subverted in Die Hard 4.0 where John McClane while being held as a human shield forces the Big Bad to shoot through a previous bullet hole McClane had already taken and kill himself.
Subverted in Kiss Kiss Bang Bang. When Perry moves in front of harry while being shot at, the bullet passes through him and hits Harry anyway.
Subverted and played straight in Django Unchained during the same scene. During a gunfight, Django shoots through a mook to hit another mook. Moments later, he is shown using the corpse of mook for cover.
In a sickening example in New Jack City, drug dealer Nino Brown uses a little girl as this when the Mafia tries to assassinate him at a party.
Granny Weatherwax implies doing this with an unconscious Diamanta when fleeing Fairyland in Lords and Ladies. She gets praised for rescuing the girl by carrying her over her shoulder, but instantly claims it was to shield herself from fairy arrows. (Since this is Granny Weatherwax both interpretations are probably equally correct.)
Subverted in Alistair MacLean's novel The Satan Bug, where the protagonist points his gun at a suspect, and then tells him not to "worry" about the Scotch tape over the barrel, since the bullets from this gun are strong enough to pierce, if necessary, even throw two persons standing one behind another.
Subverted in the book The Bourne Legacy, as a terrorist attempts to use a fallen guard as a human shield, but is mowed down when the blast go right through the body anyway.
In one of the Drizzt Do'Urden books, a drow weaponsmaster uses one of his own men as a shield when a dwarven battlerager launches himself at him from a small siege engine pointy helmet first. While the weaponmaster is still injured by the attack, the bulk of the damage was absorbed by the mook.
In Septimus Heap, when Queen Etheldredda tries to shoot Jenna and Alice Nettles jumps in between, the bullet stucks in her heart. Justified, since the bullet was primed for Jenna but the tag (the initials: IP for "Infant Princess") worked for Alice as well (since her actual name was "Iona Pot").
Inverted in Captain Corelli's Mandolin, when the Italian soldiers are executed, Carlo Guerciodeliberately steps in front of Captain Corelli and holds him there, saving his life. Also justified, as Guerciois big enough to shield Corelli completely, and his body doesn't completely stop the bullets, only absorbing enough of the force that the Captain's injuries aren't fatal.
Lampshaded in the Human Target episode "Victoria", when Guerrero uses a startled mook as a human shield against a sniper on a nearby roof. When the EMT they're protecting at the time manages to drag the mook in for inspection, he comments that the bullets passed straight through the mook's bulletproof vest, and that Guerrero's lucky they didn't hit him as well.
Happens again in the episode "Run," when Chance uses a mook both to shield himself from a guy who's firing at him from across the room, and to fire at said guy via the mook's gun. There's no lampshade this time, though.
Scorpius even compliments John on remembering this fact.
Bryce Larkin uses a human shield to get into the Intersect room in the pilot of Chuck.
Shaw uses poor Morgan as a shield to absorb a Taser strike in Season 3. Was anyone actually surprised he turned evil?
Subverted/shown correctly in Spooks when Jo Portman manages to restrain a terrorist who is trying to blow himself and everyone else up. The only way to avert the explosion is for Ros to shoot the man, however (as both Jo and Ros realize) the bullet will go straight through him and kill Jo as well; which is what happens.
In the final episode of Starsky & Hutch, Hutch is ambushed by two hitmen, one with a gun and one with a knife. He manages to take advantage of this by maneuvering the knifewielder between himself and the gunman, just in time to take the shot.
Subverted in NCIS, when the Genre Savvy human shield gives Gibbs permission to shoot through her. And he does. Repeatedly.
Played ridiculously straight in an episode of Walker, Texas Ranger. A young woman has been taken hostage and placed in front of a shotgun set to go off at a certain time. Walker finds the villain's hideout, beats him up, and drags him in front of the gun just in time to protect the girl and make the bad guy take the blast. Of a shotgun. True to form, only the bad guy is killed, when in real life, the shot probably would have gone through him, Walker, and the poor girl.
Burn Notice takes great pains to avert this. In one episode a mook has grabbed Michael, and Sam and Fiona have no way to reach him so Jesse takes a shot through Michael to kill the mook. In a later episode, Nate has the latest arc villain at gunpoint when a sniper round goes through the villain, through Nate and blows a hole in the wall behind both of them.
In "Home", Carol uses a dead body (mainly, Axel's) this way after they're killed by the Governor; Played Straight as it blocks multiple rounds from an automatic rifle without her being hit. Although they were shot in the head, and may have been wearing a bulletproof vest, since they were in one of the former guards' uniforms, but this still probably wouldn't stop all of them.
In "Too Far Gone", Daryl uses a zombie he shot to provide cover from weapons fire from the Governor's forces when they were assaulting the prison in the mid-season finale for season four, as he ran from one shielded area to another.
Bonus points in this case, as shortly before that, another Character (Bob) catches a shot which goes right through him and he explicitly demands Maggie to examine the exit wound. Obviously that only happens if you're not an enforcer of Rule of Cool (as Daryl).
Played amusingly straight in indie Roleplaying Game@ctiv8, which has a section listing how much damage is stopped by various forms of cover. A human body used as a shield provides more protection than bulletproof glass.
In Dungeons & Dragons, merely crossing swords with somebody grants you cover from anybody behind them. Justified, as the most powerful projectile you're likely to encounter is a crossbow bolt, but the person PROVIDING you said cover doesn't actually take damage from the attack they stopped.
"Cover" is defined ambiguously enough that it could be interpreted as "they're aiming at the tiny bits of you that show around their ally, whom they're deliberately trying not to hit." As opposed to "concealment", which means they're shooting through whatever is between the two of you, but are not quite sure exactly where your voonerables are.
Crysis allows the player to use Korean soldiers as shields (and improvised throwing weapons if the situation calls for it). Is reasonably justified, as said soldiers seem to be well-armoured, and won't be struggling out against you very well with the strength granted from your Powered Armor.
The Metal Gear games use both this trope and Human Shield, depending on who you capture. In Portable Ops and 3, attempting to shield yourself with a higher-ranking officer would cause enemies to hesitate, while shielding yourself with a scientist does nothing and they'll still shoot you freely.
Cyborgs in Syndicate could use a Persuadatron to make innocent bystanders follow them around and attack enemies. Since they were almost all unarmed, they were really only useful as (very short-lived) meatshields once the shooting started.
They started unarmed, but would happily pick up any loose weapons and use them on hostile targets. A crowd of 50 or so civilians even with the weakest pistol can lay down some smackies pretty quick.
Saints Row 2 allows you to use just about anyone as cover. Taking a Banger causes the others to open fire and kill their friends while you take them apart.
Enemy gang members will do this too. Unfortunately, they tend to grab random civilians rather than members of your gang, meaning there's little emotion and more "Man, having to waste random bullets". One boss fight does use a human shield to turn it into a sort of Puzzle Boss, though.
The Boss in a cutscene uses a bartender he/she has just sat down in front of when a SWAT team begins raiding the place with lethal force.
Taking a cop or civilian will cause other cops to not shoot. Unless they're behind you. They also think nothing of running you both over with a car.
Dead to Rights and the sequel have human shields as a major source of armor. Enemies don't stop shooting, but you really, really do need the shields. Really.
In Gears of War 2, Marcus may actually randomly say "Hope you're bulletproof" when grabbing an enemy for this move. Just don't try it against a Grinder. That minigun will rip the guy in half in seconds.
Both COG and Locust players in multiplayer may pick up downed enemies as a "meatshield". The possibility of using them to dissuade being fired on out of fear of hitting the hostage as a Human Shield is impossible as the move causes the downed enemy to be dead for all intents and purposes, leaving it solely to be used by players soak up more bullets. Using this slows the player down and limits them to their pistol, though the meatshield can be instantly disposed of by swapping to another weapon (and more entertainingly disposed of by snapping their neck before dropping them). Like Crysis, is reasonably justified in bulletproof-ness as the COG wear so much armour yet remain decently mobile that some players theorize it to be Powered Armor, and the Locust are scantly armoured due to having a tough hide than makes them hard to shoot dead.
A wild variation occurs in the video game BloodRayne. There, one of the titular hero's special moves is to grasp a enemy goon, bite down on his neck (she is a vampiredhampir Dhampiresa, after all), and while wrapped around him, spin him around to block enemy shots, using him as a Bulletproof Human Shieldand gaining health at the same time.
Half-Life 2 lets you do this when you have the Super Gravity Gun. After you kill an enemy with the "pick up" function, his levitating corpse makes a great temporary shield against his friends' weapons. Especially the heavily armored Elites.
All of the Splinter Cell games allow you to take hostages. Most of the time, the mooks will shoot through their friend, harming Sam. In the one instance in the second game where Sam takes the enemy leader hostage, they will circle around, terrain permitting, and still shoot him.
BioShock and Bioshock 2 lets players have fun with this: basically kill a splicer or big daddy, use telekinesis to lift body and you have an instant human shield, and when you're close enough, just toss that body at the attacker to hurt or kill them.
Victor Zsasz uses Dr. Young as a human shield partway through Batman: Arkham Asylum. The Batman takes him down anyway.
Appears in The Force Unleashed II in the "Battle of Endor" level. Starkiller uses Chewbacca as a shield against Han Solo's blaster.
Painkiller has Skulls, enemy monsters who can pick up and use lesser mooks as shields.
FEAR lets you do this, if you can get the drop on your enemy.
Time Crisis 2 had part of the final boss battle feature him using Christy as this. You're penalised points if you tag her, and if you spend more time shooting her than the boss you get a time over, but that's about it.
Operation Thunderbolt (a sequel to Operation Wolf) had this at the end; the villain used the hijacked plane's pilot. You get the Bad Ending if you kill the pilot.
In the trailer for Resident Evil: Operation Raccoon City, a member of the Special Forces is caught in the crossfire after dodging a grenade explosion. He grabs a convenient nearby zombie, which takes a few bullets for him, allowing him to run for cover.
Assassin's Creed III employs this as a combat mechanic. Connor can grab enemy soldiers and use them to absorb a volley of musket fire, then drop them to close on the remaining soldiers before they can reload. The E3 trailer also shows him using a horse for this purpose — while riding it. Even more far-fetched is Desmond being able to do this while storming the Abstergo HQ. The guards take as long to aim their sidearms as the 18th-century officers, allowing Desmind time to grab the nearest guard as a shield.
You can also do this in Assassins Creed IV, with the process streamlined from the previous game.
Jones:Seven bullets that hit my father as he shielded my mother during a robbery. They passed through him and killed her anyway.
In Borderlands 2, the Nomad Torturers carry massive, bulletproof riot shields with a hole in the center. A hole they patch up by strapping a Midget to the shield. (The Bandits are competent metalworkers, and could probably patch up the hole with a more conventional fix, but Nomads like to make Midgets suffer.)
In Chapter 11 of Max Payne 3, Max cold-cocks an AUP mook on a zipline and uses his body to shield himself from the other mooks' gunfire while he rides the zipline. At least the mook he's holding onto has body armor.
Also happens many more times in future installments. Bullets can't seem to penetrate bodies very well, but can tear through cover easily.
Agent South in Red vs. Blue picks up a soldier's corpse at one point and uses it as an impromptu shield against snipers. None of the bullets end up hitting her anyway, but even with the body armor soldiers wear, you have to wonder what good it would have done against a sniper.
This happens very early on in Looking for Group, but rather than a hero using Mooks, the Chaotic Evil character Richard smugly drags an innocent peasant into the path of several of Cale's arrows (even though he wouldn't be killed by them anyway, being undead), and enjoys the expression on his face when he realises he's killed an innocent. He even suggests going after the son and wife to make the complete set.
Cale'Anon has recently pulled this trick using Maikos. Too bad Maikos is turning mortal again.
Terra: On page 243 a mook grabs the corpse of one Alex already shot and killed and uses him to block her fire while he returns it. This set off a brief bout of Wild Mass Guessing in the comments. Eventually Holly put her foot down and pointed out that Alex never actually shot the human shield because the mook holding it was quicker on the trigger; therefore whether her rifle fire would or would not penetrate was a moot point.
In True Villains, Xaneth uses an innocent bystander as a shield against The Necromancer's attack. It's played humorously.
In Drawn Together, Captain Hero's "Hero Shield" consists of grabbing a random civilian and holding her as a shield. Despite having superpowers that make him Immune to Bullets.
In the Beast Wars episode "Code of Hero," Dinobot is under attack from Blackarachnia's eight machine guns. He grabs the already semi-bisected Inferno to absorb her last bullets before using Inferno's gun to shoot her down. Then he uses the poor Predacon's gun against him, blowing his head sky high.
Justified as Inferno, like most Transformers, is quite literally Made of Iron (or some much tougher metal).
In the murder mystery episode of Family Guy, Stewie shoots through Diane Simmons with a sniper without hitting Lois who was standing right behind her. But hey, Rule of Funny.
In an episode of Archer, Archer is teaching Cyril how to act like an agent and shoots cubes of ice at Cyril with a slingshot. Cyril grabs a nearby call girl and pulls her into the line of fire. Archer is delighted, the call girl less so.
Cheryl:(screams) Archer: Shut up! That vest is bulletproof! Cheryl: Oh. (gets shot in the arm) Ow! Archer: But it is, you know, a vest.
Justified in the Justice League episode "Twilight of the Gods." The Martian Manhunter punches his hand through the head of a robot, then uses its body to block shots from the other robots. He even keeps holding onto it after its legs fall off.
Another justified example in Unlimited. A general holds up Supergirl with a gun that fires a "Kryptonite bullet with a silicon core" (designed to spread the Kryptonite around the entire body). Green Arrow steps in front of Supergirl, pointing out that firing the weapon would use up the one bullet (by the description given, doesn't sound like the bullet would actually penetrate through the body) and leave a very pissed of Supergirl to deal with. The Question grabbed the gun away while the general was distracted.
Aside from calibre and type of firearm, different types of ammunition have characteristics that may or may not allow them to fully penetrate a human body. For example, a full metal jacketed 9x19mm Parabellum bullet may be able to go straight through a human torso, while a wadcutter .45 ACP bullet is more likely to lodge itself somewhere inside an organ (although the possibility of the bullet going out the back shouldn't be ruled out).
Expanding ammunition flattens on impact to increase the size of the wound (and therefore its lethality) while reducing its penetration, which would allow for a human body to better absorb the bullet. Soft Point Ammo has the jacket removed from the nose while Hollow Point has an internal cavity. Cruciform ammunition has an X shaped incision in the tip, causing it to split and expand in a controlled fashion. While the Hague Convention prohibits the use of expanding ammunition in warfare, it is permitted or even required in certain circumstances such as hunting or law enforcement, where quickly disabling the target may prevent loss of life and over-penetration may put bystanders at risk (though the latter point isn't nearly as significant as people make it out to be - even if your bullet won't over penetrate, many of the other bullets you shoot can still easily miss and hit a bystander—a source of civilian casualties more common than over penetration by orders of magnitude).
Frangible bullets are designed to disintegrate into tiny pieces when they impact a hard surface; this can protect against the danger of ricochets in close quarters, and limit the chance of, say, shooting through the skin of an aircraft, as well as do more damage to the body, as most criminals can't afford body armor. Air marshals would be likely to carry such ammunition.
MythBusters once did a test to see how much tissue a bullet went through (based on myths that a very fat or very muscular man survived a gunshot). 16 Inches of fat couldn't stop a bullet, nor could 14 inches of muscle. They were using a .223 round.
The reality is a little different; 12 inches of penetration in ballistic gelatin is considered the absolute minimum for a law enforcement pistol round, simulating passing through a target's arm at an angle and still wounding the heart sufficiently to incapacitate them. This is because Michael Platt survived such a shot.
Considered the minimum by the "stopping power does not exist" faction of ballistics theorists, known as the Jello Junkiesnote "Ballistic jelly" is just commercial gelatin, like that used for desserts, prepared with a different ratio of gelatin to water.. Their opponents counter with examples of subdued—specifically dead—opponents with much less penetration; since they cite coroner data they're known as Morgue Monsters.
a .223 Remington, a rifle round, could easily penetrate non-plated body armor, so the results aren't surprising.
Of course, even if the bullet or fragments of the bullet do make it through a human shield, it could still reduce the lethality of the shot.
Or increase it, as the bullet will likely be tumbling when it comes out of its first victim.
Or, in the case that it pencils straight through tissue, still retain sufficient velocity to wound and possibly kill.
BBC reporter Kate Adie was in Tiananmen Square when the army started shooting into the crowd of demonstrators. She was slightly injured by a bullet that had just killed the guy in front of her (and possibly others).
In David Simon's Homicide: A Year On The Killing Streets, Simon relates the story of detectives who were called to investigate the death of a man shot thirty-five times and the wounding of a man shot four times. It was ultimately revealed that a drug dealer and his muscle had gone to meet with another man. When an argument occurred, the dealer ordered his muscle to open up on the man with a MAC-10. Thinking fast, the man grabbed the dealer as a human shield, and the muscle ended up riddling his own boss with bullets. Only four passed through to wound the man, who survived the ordeal.
The FN 5.7×28mm bullet (used by the sci-fi looking FN P90 and Five-seveN) deserves mention because it was designed to expend all its kinetic energy in the first person it hits, so if it hits your human shield in the center of mass, you're going to be okay.
Some soldiers will attempt to do this in certain situations, with results being varied.