This is when a weapon is used in such a way that shouldn't work (or would simply be inefficient). It can be broken down into a few subtypes;
Where the writer, actor or animator didn't bother to find out how the weapon is used (e.g. Using a rapier to slash, using a mounted weapon as a man portable weapon).
When the weapon does something it wouldn't normally (see Set Swords to Stun for the most common example, lethal weapons being non lethal) or works better than it should (e.g. destroying a tank with a pistol).
Using the weapon in the place of a tool when the tool it's replacing would do the job more efficiently (tools which are much more effective as weapons than they should be also fit).
Using a weapon which shouldn't be available (e.g. being anachronistic or extremely rare and/or expensive).
Using a weapon which wouldn't work in real life (e.g. BFS which are too heavy to lift).
Using a weapon for its intended purpose, but also using it for other purposes that it wasn't intended to as a secondary attack (e.g. using a bow as a close quarters melee weapon or the hilt of a sword for a blunt weapon).
While it's possible to swing the blade of a kusarigama by the chain (Wikipedia calls this the "Houten Ryu style"), doing so has a tendency not to work very well, and the chain is just used for swinging the weight around. Except in fiction. For this reason fictional examples tend to have smaller or non-existant handles.
All swords tend to always be used to slash. Even specialized swords which might be designed for other styles (for example rapiers are designed for thrusting and sabres are designed for slashing).
Although they are designed primarily for thrusting, many rapiers do retain a sharpened edge. The resulting cuts are rarely more than a distraction or nuisance, but a well-timed slash to the head above the eyes can be quite decisive when your opponent is blinded by blood, not to mention if you can manage a draw cut that damages or outright severs a tendon.
Similarly, there's an illegal move in boxing called "the sweetener" where you pretend to be throwing a hook, and hit the opponent's forehead with the bony part of your elbow (elbow-blows are not legal in boxing)—it's quite likely to break the skin, and he'll get blood in his eyes. It's also difficult to tell if a hit like that was intentional.
Conversely, it's common to see somebody get impaled on all variety of swords, even ones that are designed with stabbing as a secondary function at best.
Kunai are pretty common targets of this trope. Kunai are often portrayed as having sharp edges. In actuality, only the tip is sharp. The kunai was designed as a trowel, not as a dagger. Following that, they were not typically thrown (but then again, neither are most weapons that get thrown in fiction, and throwing a kunai is certainly more plausible than throwing a sword).
Using two handguns at once. You can fire them that way, but aiming is impossible and since you're only using one hand each the recoil sends your bullets all over the place. Also, using high caliber pistols like a Desert Eagle with only one hand requires an extremely strong arm, as the thing weighs almost 2 kg, and that's not even considering the recoil.
To say nothing of the fact that the human eyes aren't designed to move independently, so looking down the sights of two handguns at once isn't going to happen. Even if a character is superhumanly strong and the recoil isn't a problem, the best they're going to manage is pointing in the general direction of the targets and hoping they hit. A reptilian character, on the other hand, might not have this problem, as some reptiles do have independently-moving eyes.
Shooting from the hip is pretty much pointing your gun in the general direction of your target and firing without aiming at all. With only 30 rounds in the magazine even an assault rifle has a very good chance to not hit anything at close distance, especially when you swing it around in an arc. The only valid purpose of doing something like this is to get the enemy to duck behind cover instead of shooting at your comrades while they maneuver. That being the main purpose of light and medium machine guns (which also tend to have either large drum magazines or even larger ammo belts to sustain this kind of fire longer than an assault rifle could) in infantry combat.
It subverted the "used the blunt side" trope, though—Zelgadiss assures someone he's just cut that he didn't use the cutting edge, then pauses, and looks at his sword. "Wait...this is a double-edged sword. Uh, sorry?"
Death The Kid in Soul Eater holds Liz and PattyThompson upside down and pulls their triggers with his pinkies. This might be because he uses them something like tonfa in close combat (so holding them the way he does would give him more torque) but that just justifies one example with another.
Being a Humanoid Abomination also justifies his improbably weapon skills. Sure, a human can shoot a pistol with their pinkies, but mostly just with one gun and with both pinkies on the trigger. Two guns that can transform into even larger Hand Cannons (the largest forms needing stands to support them) and with crazy aiming skills? Only a god (which Kid is) could pull something like that off.
Gundam SEED: Kira and Cagalli are attacked by terrorists while out on shore leave. Kira comes across a pistol, and promptly uses it...by throwing it at one of the terrorists, then performing a flying kick in the follow-through. Lampshaded by Cagalli: "Do you know how a gun is supposed to be used?!"* He doesn't.
Kaede Nagase in Mahou Sensei Negima! is a Ninja who mostly follows the rules of Hollywood Ninjas (kunai, explosives). Except for her being listed on BFS; it stands for BFShuriken. She does throw it, but it's normally used to shield her and smash her targets. Note that shuriken were used as make-shift blades for close-in defense in Real Life. They just happened to often be small and annoying.
On one occasion she does throw it, she takes out a number of opponents. She assures some onlookers that the giant whirling deathball she just threw is totally nonlethal because she hit with the back of the blade. Do note that her shuriken doesn't appear◊ to actually have a dull side, and that the onlookers still think it should be fatal.
a completely legitimate tactic in real life, but it's not recommended because that's why you have a knife
, which doesn't even qualify as Pistol Whipping. After almost shooting himself in the crotch through that method, he realizes how it's supposed to be used (but still holds it upside down). Later, during his first fight against Viral, he more cleverly uses an arrow as a makeshift knife.
Subverted by Golgo 13. His choice of an M16 seems an odd choice for a sniper, especially since popular imagination has it to be some kind of close combat sub-machine gun. However, as the creator is more than willing to point out, the M16 is an assault rifle, and is perfectly fine as a sniping for target up to 500 meters, especially in the hands of a skilled user more interested in getting a job done than setting distance records…which Duke Togo certainly is. It's also a very easy gun to both find and modify, which is handy for a professional who discards weapons as soon as he's through with them. An M16 can also be made very accurate indeed, at least within the range limitations of its 5.56mm round.
Ben Beckman, a member of Shanks' crew in One Piece, wields a flintlock rifle exclusively as a bludgeon.
Captain America uses his mighty shield as a throwing weapon. And as a club, but that's actually an often overlooked Real Life application of the shield (it's traditionally a bloody great lump of wood with metal furniture. Why wouldn't you hit your opponent in the face with it?).
Cap's shield has some special qualities that make throwing it a great idea, mainly due to being made of the fictional super metal Vibranium. Though only Cap and a small handful of other people are capable of throwing it without being more of a danger to themselves than to the enemy.
Note that the shield's edge is sharp- he once used it to decapitate a vampire!
Cap's Avengers teammate Hawkeye is the world's greatest archer, but when cornered he can easily use his bow as a melee weapon. Once when his bow was broken by enemies, he managed to subdue them by throwing arrows like darts.
In The Mummy Returns, they have flashback sequences to ancient Egypt, where two female characters duel with dual sai. Not only did the sai originate as a weapon in around the seventeenth century AD in East Asia instead of 2000 BC Egypt, but sai were typically blunt truncheon-type weapons mostly used for blocking and breaking swords instead of sharp knife-type weapons. Although there is evidence of sai-like weapons from ancient Egypt which were blunt, it's not likely they were used like that.
In Ultraviolet, guns are used for everything, including opening doors, cauterizing wounds, and occasionally killing people.
In Predator, Jesse Ventura uses a hand-held GE M134 Minigun. It is impossible for anyone to really hold and use this weapon. In Real Life this gun is mounted on a helicopter or armoured vehicle, it is far too heavy to be used otherwise. An even smaller version was actually designed to be man-portable, the XM214 "Microgun" of the 1970s, but even it was designed to be used with a tripod, and never made it past the prototype stage.
More specifically, a big guy like Ventura can certainly lift the 70-pound Minigun, but carrying a weapon that heavy into battle in addition to all the usual equipment that any soldier carries would be quite impractical. And also carrying the 2,000-round ammo drum (the ammo alone would weigh over a 100 pounds) and power supply on his back would've been completely implausible. Because even standing while carrying that much weight would be difficult, Ventura carried less than a full drum of blanks (which weigh less than regular ammo) and the battery was on the ground, hidden from the camera's view. But the main problem is the recoil, which would be far beyond even the strongest human's ability to handle. Again the blanks helped with that (since blanks have much less recoil), as did slowing the rate of fire dramatically, which also made the scene more dramatic by making the barrels spin slowly enough for the human eye to follow.
In Terminator 2: Judgment Day, the T-800 uses the same GE M134 Minigun. The Terminator's use of it can be handwaved by being a super-strong cyborg, though one wonders why Sarah Connor kept a helicopter's weapon in her arsenal to begin with.
Underworld is notable in the instance where Selene uses a pair of fully-automatic pistols to shoot a circle around her feet, creating an exit through the floor in a building when she tries to evade the werewolves and retrieve Michael.
In The Avengers, Hawkeye runs out of arrows and is forced to fight the Chitauri with just his bow instead. This one has some historic precedent - unstringed longbows, for instance, made pretty good quarterstaffs.
In Hot Shots! Part Deux, Topper's machine gun runs out of ammunition. He spies a crate filled with bullets and throws them at the bad guys...killing them.
The Toolbox Murders remake and The Sleeper both have scenes where characters have their throats slit with the claw-end of a hammer.
Soviet troops in Kingdom Of The Crystal Skull use Chinese Type 54s (copies of the Tokarev TT-33), Type 56s and 56-1s (copies of the fixed and folding stock AK-47, respectively) and a Type 69 grenade launcher (copy of the RPG-7), as well as a Browning M1919A6 with the stock removed to mimic a Soviet KPV machine gun. Soviet troops never used Chinese weapons, nor American.
In Discworld The City Watch Novels, Detritus's the troll Weapon of Choice is a ballista. He is able to use it like a crossbow just because he's so damn big. Sometimes it's used to fire the iron spears it was designed to fire, but most of the time Detritus loads it with bundles of arrows. Which shatter from the force of being fired. Then burst into flames from the air friction. When Detritus test-fired it for the first time, it wiped out the training targets, the hill behind them, and some birds that happened to be flying directly above him. He calls it the Piecemaker.
It cannot be fired indoors. Primarily because it's no longer indoors after he fires.
In the Humanx Commonwealth novel The End of the Matter, Anti-Hero Skua September puts in his first appearance wielding a heavy military laser designed to be fired from a tripod. He wears the massive power supply as an impromptu backpack. He does comment that it's awfully heavy, which says something coming from a man as huge as he is.
In Charlie's Monsters: Nightmare Academy, Charlie uses his rapier to slash at something.
In The Incredulity of Father Brown, this is actually the answer to the death of a millionaire that seemed to have been shot through a window by God. He was stabbed with an arrow where the angle pointed to the window.
MacGyver once used a revolver chamber as an improvised wrench, 70 feet up in the air. He even stated that he is afraid of heights and hates guns, and he happened to be in this situation.
Robin Hood the BBC version: when the Sheriff's men are at a distance, the Outlaws shoot them with bows; when the Sheriff's men are close, Outlaws use their bows as quarterstaffs and hit them. The whole point of a staff is that it is solid; the whole point of a bow is that it bends.
Lampshaded in Merlin when Merlin, who in this version is a peasant boy about the same age as Arthur and realistically does not swordplay, just smacks the guy with the blade. It works, and Arthur's reaction is priceless.
The Mordhau in German School of Swordsmanship. It is basically grabbing the sword on the blade, swinging it wrong way around and smashing the opponent's face with the crossguard. Likewise, the crossguard can be also used on tripping the opponent.
In the German School of Swordsmanship, the whole sword is a weapon. The pommel doubles as a mace, the crossguard can be used as a weapon or tripping or binding instrument, and another unusual technique is Halbschwerten, which means grabbing the sword with one hand on the hilt and another on the blade and cross-checking the opponent.
Swords designed to be held by the blade often have a false edge half way down one side or near the hilt specifically for the user to grab (some larger swords can be used like polearms this way).
Any form of sword fighting intended for use in real combat (as opposed to tournament fighting or acting) has this sort of thing. You're trying to kill or disable the other guy before he's able to kill or disable you, and you may be trying to deal with multiple opponents or confined spaces where some types of moves don't work. If you lack the room to swing your sword and have someone up close, striking them in the face with the hilt can be quite effective (most swords used in real combat would, used this way, be like being hit with a hammer).
Real Life: on MythBusters, shooting an escape hole in the floor or shooting out a lock proved possible but far, far more difficult than using the proper tools - a boltcutter and circular saw, and required fairly impressive firepower.
SWAT and Special Forces personnel called upon to breach a door are trained to shoot at the hinges instead of the lock as they tend to be flimsier, but even then, without specialised ammunition they're normally better off with a sledgehammer or crowbar.
Carlos Hathcock, a famed Marine Corps sniper (one of his famous achievements being a Scope Snipe), once used an M2 Browning .50 machinegunmodified and mounted with a scope◊ as an improvised sniper rifle. He set the record for the longest combat kill at 2,286 meters, a feat not surpassed until the Afghanistan war, in which dedicated sniper rifles firing the same .50 caliber round were employed. This counts as a trope example since the M2 was not designed for precision sniping, but does have adequate range.
His successor is Corporal of Horse Craig Harrison, who took down three separate targets with a single bullet to each with an L115A3 sniper rifle from 8120 feet away, or 3200 feet beyond the rifle's effective range.
The IMI Galil had a bottle opener built into its bipod because of problems with soldiers using magazines of other rifles as improvised bottle openers, damaging the magazine lips in the process and causing feed problems.
That is exactly the reason why the Finnish army RK62 magazines were re-designed to have sturdier lips...
On a similar note, the Russian Makarov pistol makes for a great bottle opener when the slide is locked back, as there's just enough gap between the frame and the barrel to fit a cap in.
The US Army had the same problem.
Before the invention of the Bayonet, the only thing musket users had when the enemy got close or they were out of ammo was to use their guns as clubs. Many were fitted with a metal piece on the end to make sure they are very good clubs.
The British-designed Sten Gun, particularly the wartime Mk2 version, was notorious for not just discharging but emptying its entire magazine if dropped. It was also extremely cheap and easy to make. Partisans in occupied Europe would sometimes turn this bug into a feature; dropping a spare one out of an upstairs window into the midst of a German patrol was a pretty good field-expedient grenade substitute, and one that could usually be reloaded and used again afterwards.
There are a ton of video games which feature vehicle-mounted — and electric-powered — miniguns being carried and fired by individual humans on foot, with no extension cord to power it or source of ammo to shoot from it visible anywhere.
Prototype 2: James Heller is capable of ripping weapon mounts off of tanks and helicopters and carrying them around. In universe he's strong enough to punch out a tank, so this makes sense. However, in addition to carrying these weapons around and throwing themhe can actually fire them which qualifies for this trope since these are vehicle mounted weapons that fired from inside the vehicle and don't actually have triggers. Many of these weapons (especially helicopter missile launchers) seem to have far more shots than you would expect from looking at them, which also counts.
Final Fantasy series (the older ones, anyway): The Ninja's Throw technique makes you able to throw weapons at the enemy. All weapons. Yes, even katanas. And it's even more effective depending on the power of the weapon. So, throwing a katana is more effective than throwing a dagger, and some weapons are only effective when thrown, like the Excalipoor.
The same issue is present in both Neverwinter Nights games. You can certainly use a rapier...but it has the same attack animation as a longsword, and the result looks as suitably ridiculous as you'd imagine.
In God Hand you can get the tremendously useful rocket launcher... which you use with the same animation as every other weapon and use it to beat someones head in. After you've fired it's single payload of course.
Toshiie Maeda can use the two spears he carries on his back as boomerangs, just as his lord Katsuie Shibata can with his axes.
In Dynasty Warriors, Zhong Hui uses five floating swords that he controls seemingly through some form of telekinesis, although in the game anyone can use the swords like that.
Fist Of The North Star Kens Rage gets in on this with Jagi, the Dirty CowardUnfavorite of the Hokuto brothers. Instead of using his fists like pretty much everyone else, he elects to use shotguns and a freakin' bazooka as his weapons of choice. Sometimes this involves firing them like a proper gun-toting maniac. More often, though, he's clubbing people with them, or swinging the bazooka around like a club to wallop his enemies into the distance. Oddly enough, his regular attacks simply leave normal corpses behind, but his special attacks cause the series' traditional cases of Your Head A Splode.
The sniper's taunt kill with his unlockable bow has him stab an opponent with the arrow.
Another of his unlockables is a tranquilizer rifle, which he uses to fire piss-filled darts. They're as deadly as regular bullets, except they can't headshot.
Saki from Suguri also uses a grenade as a melee weapon. It has a 25% chance of exploding and dealing double damage.
Samurai Shodown has a bit of this in its repertoire, of course. Some are justifiable, like the swordsmen taking a cheap shot with the hilt (or, for Ukyo, with the sheath). Some are a little more Rule Of Cool, like Earthquake tossing the blade end of his kusarigama. And a few are less than textbook, like Cham Cham using her boomerang as a melee weapon.
In World of Warcraft, the Warrior ability Heroic Throw does exactly what it sounds like it does - no matter what size or shape the Warrior's weapon is. One tie-in novel has a (granted, extremely skilled and demigod-blessed) warrior throw his giant two-handed sword like a spear.
The ability Titan's Grip allows a Warrior to Dual Wield two-handed weapons. It gets a bit ridiculous when a gnome dual wields giant battleaxes.
The latest version of Dwarf Fortress brings in the German School of Swordsmanship mentioned in the Real Life section above, enabling Adventure Mode players to club opponents with pommels, the flat of swords and the shafts of polearms.
The Super Smash Bros series has a multitude of ranged and melee weapons that can be used in a normal fashion... or you can simply opt to throw them into your opponent's face instead. Amusingly, this frequently is a more effective method of K Oing your opponents then using the weapon in the standard fashion, due to it causing more damage and/or being a lot easier to hit them in this fashion. The Home Run Bat is probably the best example, as while being wielded, the only notably powerful offensive option it has is its ridiculously slowOne-Hit KO smash attack, whereas it can be turned into a speedy projectile with a strength rivaling that of many fully charged smash attacks when thrown.
The introductory mission of Battlefield 3 has a shotgun used to bar a door between the cars of a subway train. You don't even get to use it for its intended purpose, as five seconds after you remove it from the door an enemy ambushes you and throws it out the window.
Intrusion 2: Where to begin with Maku, she uses a laser cannon twice as big as her, twirls it around like a baton, swings around on a grappling hook while shooting it, uses it as some sort of jetpack, fires it at a disco ball to reflect and split the beam, and fires it into the ground to make molten lava shoot out of the ground.
Chaos Fighters has swiping, an type of attack which the weapon (usually a sword or a lance) is swung in a direction perpendicular to how its blade is alligned. In reality this would not work as well as depicted and potentially breaks the weapon.
An episode of The Simpsons had Homer buying a revolver, which he promptly uses to do things like opening beer cans and changing TV channels. This recklessness causes Marge to leave with the children and gets him kicked out of the local NRA.
"Honey, a gun is just a tool, like a screwdriver, or a crossbow, or an alligator..."
Of the four Teenage Mutant Ninja Turtles, only Leonardo comes close to fighting appropriately with his weapon. Donatello is decent, but never uses half the techniques for a bo, Michelangelo is missing 95% of his nunchuku skills since he dual wields them, and Raphael uses his sai as knives instead of defensive bludgeon tool.
This varies somewhat by work: in the 2012 series Raphael actually uses his sai to catch and break weapons (and limbs), although he still occasionally uses them to stab as well.
On Family Guy, when Peter becomes a professional jouster, he tries to fish toast out of the toaster and butter it with his lance. It works about as well as one might expect.