1066 and All That explains that one advantage of the Roundheads having perfectly round heads was that, "if any man lost his head in action, it could be used as a cannon-ball by the artillery (which was done at the Siege of Worcester)."
Bas-Lag Cycle: A race of Cactus People known as the cactacae, while they can be punctured by bullets, crossbow bolts, arrows and the like, have a complete lack of internal organs that makes such weapons next-to-useless. They're also enormous, extremely strong, and covered with spines, which makes close-range weapons like blades or clubs viable but generally a bad idea. A sort of crossbow called a "rivebow" was invented to get around this problem. It fires huge whirling chakris that can sever the heads and limbs of humans and cactacae alike, but rivebows are so heavy and unwieldy that usually only other cactacae carry them.
Un Lun Dun: Deeba acquires the UnGun, which fires larger amounts of whatever you put in it. It, among other things, fires hair and ants. This is WAY more badass than it sounds and then it fires nothing...uh, well, more like "unfires," acting like a vacuum to suck up the Smog.
Dr. Thorndyke: In "The Aluminium Dagger", the specially-made titular weapon was shot out of a Chassepot rifle to create one of the most far-fetched locked-room murder mysteries yet.
In The Chronicles of Amber, the only substance that can be used as a propellant for firearms in Amber is jeweller's rouge.
In Dance of the Butterfly, enhanced ammunition is used by Hunters to harm supernatural creatures. The bullets come with a tracer-like amber coloring when fired to illustrate their special nature.
Dark Future: Explosive rounds are remarkably widespread in everything from pistols up. GenTech manufacture a special version of napalm that genetically bonds to skin on contact and continues to burn underwater and inside of people. They also make smart bullets, referred to in-universe as 'smugslugs,' that can track human heartbeats.
Deception Point: The evil Delta Force soldiers carry guns that can make ammo from nearly anything you jam in the barrel, from ice to sand.
The Demon Princes puts in the hands of its protagonist a device that fires "slivers of explosive glass" and either another device or the same device with different loadings/settings, which discharges very fine needles that cause intense and prolonged itching.
Discworld: Detritus the Troll uses a converted siege crossbow loaded with a bundle of regular crossbow bolts. The firing speed is high enough that the ammo generally shatters and then bursts into flame (or vice-versa) ending up in a supersonic flaming ball of wooden shards, which is why it's called "the Piecemaker". The Watch typically has Detritus pull it out when they need a door blown off its hinges and collateral damage isn't a concern.
The Dresden Files: After seeing gun-for-hire Kincaid use dragon's breath shotgun rounds — a real-life form of magnesium-based incendiary shotgun shell that produces an effect similar to a very short-term flamethrower — Harry does a little research and finds that shotguns can shoot all kinds of interesting things. He settles on the fireball shell — which doesn't exist; he's probably referring to the flare shell, which does and is similar — for himself.
In one book, Harry is fighting some vampires and finds himself needing to deal with an Entropy Curse. When he successfully redirects it, he finds one of his enemies being targeted by a frozen turkey. Specifically dropped from a passing airplane, even. The whole battle stops so everyone can process what just happened. (And earlier in the book, someone was hit by a car. What makes it "abnormal" is the target was waterskiing at the time.)
Dungeon Crawler Carl receives a loot box from a benefactor, containing a xistera extension customised to throw decapitated love doll heads. He immediately puts it to use.
In The Holmes-Dracula File, the Count made a point of congratulating Holmes for thinking to use wooden bullets. This one is a fairly common strain of Abnormal Ammo. It's the secret weapon used to tip the balance of power between warring vampire factions in the film Sundown: The Vampire In Retreat, while the vampire-hunter squad in the excellent TV series Ultraviolet (1998) use a high-tech, hardened-carbon variant.
In another Saberhagen vampire book a contemporary police officer improvises a wooden projectile by sticking an ordinary wooden pencil into the barrel of his revolver.
Goblins of the Labyrinth, a tie-in book, mentions a goblin tradition that the first shot of each battle should be harmless but unpleasant and shows a goblin clearly attempting to crap into a cannon barrel. It also has a kind of wine that requires the sharpened cork to be fired like a cannonball, leading to considerable casualties, especially since the wine itself is deadly poisonous.
John Carter of Mars has breakable glass bullets, filled with a powder that explodes in sunlight. The Literary Agent translates the Martian term for the powder as radium, but is stated that it's because it's what he expects from it anyway.
Krabat: Golden bullets are the only ones that can kill magic users. At one point, a golden button is used as a bullet.
Kingdom Of Jackals: Bullets are made of glass, not metal, because their propellant is explosive tree sap rather than gunpowder. In Nature, such sap is produced by boom-barrel trees to launch their seeds for miles; in firearms, the two sap components are contained in rear chambers of each cartridge, separated by a thin partition that's shattered by the trigger-mechanism to make them mix and explode. Disadvantages to this system are that 1) multi-shot weapons tend to blow up in wielders' hands because shock waves from a first shot can rupture the adjacent cartridges, and 2) misfired rounds ejected to clear one's weapon become inadvertent land mines, ready to blow your foot off if you accidentally step on them.
Known Space: Agents of ARM generally use guns that shoot crystallized doses of fast acting sedative. How or why this is better than tranquilizer dart guns is unknown. Probably it's better because the ammo is smaller and lighter. A dart gun has to shoot not only the tranquilizer, but also the syringe that delivers it; a "mercy pistol" only has to shoot the tranquilizer itself.
In Logan's Run, the Sandman cops carry The Gun, which is a 6-shot revolver where each round is different. Among its payloads are a regular bullet, an expanding net, and a heat-seeking bullet. Oddly enough, they don't seem to carry backup rounds...
In The Lord of the Rings, the forces of Mordor used catapults to launch the severed heads of their defeated enemies over the walls of Minas Tirith, mostly to mock and demoralize opponents.
Mistborn: The Original Trilogy: Mistings prefer to use their pseudo-magnetic powers to launch coins at enemies — and to achieve Not Quite Flight by firing them into the ground and pushing off to shoot themselves into the air. This gives them some aspect of inconspicuousness, as carrying a coinpouch is hardly suspicious; plus, coins are the most common metal thing of suitable size and weight to use for ammunition, since the local Evil Overlord has suppressed the development of firearms. By the time of Wax and Wayne, society has rediscovered firearms and regular bullets are back in vogue. Also in Wax and Wayne, Wax (himself a Coinshot Misting) wears a vest with mostly wooden buttons and one steel button as an Emergency Weapon.
Nightside: The Speaking Gun fires lethal words... specifically, the Words of God, inverted. Whatever God created, the Gun can unmake by speaking the Word of Creation that brought it into being, backwards.
The Order Of Odd Fish: The Apology Gun shoots... well, apologies. However, they can range from extremely sincere to lethally sarcastic.
Poul Anderson: At least one short story involves tranquilizer darts that, if they hit a wall or armor instead of flesh, would break open — and then the drug inside would instantly volatize into tranquilizer gas.
Prince Roger has "bead rifles", which use mass driver technology to propel glass beads at hypersonic speeds. The energy release at impact is very destructive. Glass beads are cheap, easy to make, extremely hard, and tend to shatter on impact so you don't need to worry about over-penetration. A mass driver will presumably let you fire them without breaking. Even more exotic is tightly coiled net made of monomolecular filaments that expand upon firing. They make mincemeat out of unarmored targets.
Rick Brant: In The Pirates of Shan, Zircon fires tacks out of a cannon at the eponymous villains. Later, Chahda sprinkles the leftover tacks on the deck of their ship to injure any pirates who try to sneak aboard at night.
Riley McDaniels: In Discovery at Flint Springs, Aaron uses his plane to drop flour bombs on a group of fossil thieves
Sharpe: In Sharpe's Revenge, Sharpe is severely outnumbered (as usual) but does have a chest of gold coins, which he fires at the approaching enemy. This is not to kill them, but to get them scattering to pick the coins up so he can escape.
Sigma Force: In The Seventh Plague, Kowalski uses an experimental gun called a 'Piezer' that is described as a sawed-off shotgun... that fires electrified crystals, delivering a powerful shock to anyone and anything it hits.
Sir Henry Merrivale: The Plague Court Murders involves a murder where the victim was shot by a bullet carved from rock salt that dissolved in his body, leaving no trace.
"Some Knowledge of the Knife", a J.T. Edson short story, us a murder mystery in which the assassination weapon is an oddly-balanced knife fired from a large-bore "wall gun".
Star Carrier: H'rulka ships carry guns that fire what amounts to a miniature black hole at their targets. The weapon can easily one-shot smaller Confederation vessels at longer range than most of them can return fire from.
Star Wars Legends: Every ranged Yuuzhan Vong weapon fits this description, from the living ship that fires miniature black holes to a living snake/staff/whip weapon that spits venom.
In The Strong Man, the audience watching Harry Langdon's strong man act turns into a rioting mob. Langdon throws the weights and barbells from the strong man act into the cannon meant for the Human Cannonball finale, and fires at the crowd. He also uses a barrel of liquor as ammo.
In The Thackery T. Lambshead Cabinet of Curiosities, the Bear Gun is mentioned at the very end. It shoots bears. Read that again. Tiny bears are used as the ammunition, which expand at some point after leaving the barrel. On the same page as the Bear Gun is the "Coffin Torpedo", which is based off a real life device to prevent desecration of one's coffin. However, instead of powder and shot, this coffin torpedo is implied to employ a nuclear device.
Soledad O Roark: Soledad goes so far as to design her own ammo for fighting Mutants, with an average of one Achilles' Heel exploited per ammo type. A few of the many examples include phosphorus bullets to fight pyrokinetics, bullets coated with contact poison for foes that are Nigh Invulnerable homing bullets for use against enemies with Super-Speed, and exploding bullets for virtually anything.
Soul Drinkers: One weapon is a daemon bound into a gun that fires its own daemonic spawn at enemies.
Thousand Sons: In The Tale of Ctesias, the titular sorcerer owns a bolt pistol whose rounds contain vials of liquefied daemon essence instead of the usual explosive charge. Firing one causes the vial to shatter, unleashing the daemon trapped inside and allowing it to attack the first thing it sees.
Valhalla by Ari Bach features robotic knife insects, microwave weapons and gatling shotguns, and mentions drill-shot, flesh eating bacteria injectors, grinding needle disks, deep-tissue spaz-razors, chainsaw launchers and many even more bizarre weapons.
