In the relentless cheerfulness of the far future, the weapons used by the Tyranids are, just like their armies, home-grown. These weapons are creatures in their own right, inseparable from their hosts and are counted as a single bio-organism. Some of them fire a "dumb" projectile like a spike, whereas other use seeds or even acids and living creatures. The downside to this is that bio-weapons are incapable of the explosive firepower that Imperial, Eldar and Tau guns have. They all trade this disadvantage in for being Assault weapons, meaning they can be fired on the move. Also, the explosive firepower is instead traded for more.... interesting and strategic effects. Another note is that due to their appearance, Tyranid weapons are some of the most badass and phallic looking in the entire of 40k.
Many of these weapons can be used by creatures of different sizes, but for the sake of simplicity the weapons have been ordered by the smallest creature that can use them. Small creatures are the equivalent of Gaunts, Medium are those comparable to Tyranid Warriors and Biovores, and Large are those akin to Tyrants and Carnifexes. Then there are two other categories: the Inbuilt weapons, often part of the upper bodies of the largest organisms, and the Melee weapons for the more choppy inclined creatures.
Small weapons are used by Termagaunts, the working class of any Tyranid army, and Gargoyles.
The Devourer is simply put a lump of flesh that launches a shower of worms onto a target that immediately start to burrow into the target, crawl up to its brain and devour it alive. This weapon has a nice range quite dakka for its size, albeit it doubles your Termagaunts in cost (while tripling their number of ranged attacks per round, a very economical way to increase dakka as long as you protect your Gaunts somehow). If you take this weapon in large broods, you will be aiming at your enemy's biggest units to just drown the suckers in dozens of shots.
The Fleshborer is one of the most iconic Tyranid weapons, a simple design that fires a borer beetle that eats its way through anything it comes across during its flight. Being the basic weapon for Termagaunts, stats equal to the Bolter aside from half range and being an Assault weapon, this will prove to be more than enough to kill any unit of IG, Orks, other Gaunts or even basic Eldar units. Range 12, strength 4, AP 5, and it is an assault 1 weapon.
 Spike Rifle
The Spike Rifle is a weapon exclusive to Termagaunts. It is, simply put, a harpoon launcher. That's about it. Statwise it's less imposing than it's description: range 18", S3, AP-, Assault 1. Only use these if you do not want to pump a huge amount of points into Devourers but you do not want half your brood to not be able to fire when the Devilgaunts do.
A lighter weapon, the Spinefists are used in pairs. They shower the target with a hail of poisoned spikes, the sheer amount of darts fired (as in: twin-linked) ensure that the target is hit. A tube runs from the air sac powering the weapon, through the user's arm to its lungs. This means that larger creatures get more mileage out of what is essentially the same weapon, proving that even something as simple as a blowgun can be horribly perverted by the grim darkness of the far future. Range 12", S3, AP5, Assault X, Twin-linked (where X is the number of attacks on the wielder's unmodified profile) The -only- time you want to take Spinefists is if you have to use up a few more points and want to give your Raveners or Rippers a ranged attack before they pile into close combat. All other models have better options for close to the same price.
With this weapon, your Tyranids too can learn how to shot web. The effects of the Strangleweb can be described best as a non-cutting variation of the Death Spinner used by the Warp Spiders: instead of slicing through their opponents it constricts them, and maybe crushes them to death. That's a pretty big maybe: wounding even basic Guardsmen at 5+ and rolling against Strength instead of Toughness to wound means that this weapon would be best if used against the likes of the Death Guard... if shooting them wasn't a stupid idea for a Tyranid player in the first place AND if you could take more than 1 of them per 10 Gaunts.
Medium weapons are used by Warriors and organisms based on the -Vore and -Guard body archetypes.
 Barbed Strangler
Remember that 70's cult classic Zardoz? Sean Connery dressed in red gimp getup and killing dudes like he's Khârn, at the behest of a giant floating head that claims that "The gun is good, the penis is evil." because the penis shoots seeds that create life? Well, the Barbed Strangler chooses to be both. The gun is very basic, a muscled tube that stores its seeds in a sack at the base, and shoots them with a powerful spasm. The seed upon impact greatly expands and sends tendrils to rip and tear through anything that gets in its way. Using the large blast template, you can cause a lot of damage to a blob in a single shot.
The traditional weapon of the Warrior, the Deathspitter works by stripping a large maggot of its shell and hurling it at the enemy. Its huge guts are highly corrosive and can splatter several enemies at once. Serving as a decent weapon against MEQs, the weapon relies on its rate of fire to deal damage.
 Flamespurt Cannon
It's a Heavy Flamer. Smarter than its host, the Flamespurt Cannon serves as a bodyguard while the Pyrovore om nom noms the delicious biomass and make it ready for easy digestion. Instead of working together, the weapon and the beast function apart from each other, so both can dedicate themselves fully to their tasks. If they were used, anyway.
 Impaler Cannon
The Impaler Cannon is your answer to anything up to AV 12. This beautiful gun fires a bony spine, steered by a shard-beast at the base of the spine, whose innards are ripped from its body when the spike is fired. The spines are launched with enough force to rip through man and machine alike. Though its range is limited you do not have to actually see your target to be able to shoot it, without a significant downside. Oh, and it has a higher rate of fire than the Imperium-equivalent, the Missile Launcher. What's not to like? This is your go-to weapon for dealing with METAL BAWKSES.
 Shock Cannon
Used by Hive Guard, the Shock Cannon shoots out a claw over a considerable distance which latches on to its target and delivers a powerful bio-electric shock, disabling or even destroying a vehicle.
 Spore Mine Launcher
Created as a result of fights against the Imperial Guard, the Biovore displays this by being the Tyranids' closest thing to an artillery piece and having a massive scrotum befitting a Guardsman's huge brazen balls. Armed with the Spore Mine Launcher, it can lob Spore Mines a considerable range. Though not as deadly as most other artillery weapons, the Spore Mines are unique in the way that if they don't land too close to the enemy, you can deploy them onto the battlefield as living mines, serving as an obstacle for your enemy that must be either moved around or shot. Generally more useful in larger point games or Apocalypse.
 Venom Cannon
Seen as the primary anti-tank weapon, the Venom Cannon launches a hail of corrosive crystals by way of a biological railgun. Though not as effective as you'd wish the Venom Cannon can cause quite some hurt on Aspect Warriors and the like. It is ineffective against heavy tanks but okay against transports and open-topped vehicles, though its primary targets should always be medium infantry.
Here the biggest of weapons are described, used by the biggest of nasties.
 Acid Spray
Yo dawg I heard you like Hellhounds so I put a discharge of digestive fluids that works just like a Hellhound's weapon on your Tyranid MC and upped its cost by 100+ points. That's pretty much the gist of the Acid Spray. The problem is that you have a lot of weapons with stats like that, so using an expensive MC that attracts fire like there's no tomorrow is a bad idea.
 Brainleech Worms
The biggest of Tyranids have devourers loaded with Brainleech Worms instead, a species larger and even more aggressive and hungry than the regular devourer worms. Range: 18", S6, AP-, Assault 6. If you're not using two sets of Twin-Linked Brainleech Devourers on your Flying Hive Tyrant and any Carnifexes you're fielding, you're doing it wrong.
 Fleshborer Hive
Think the Leman Russ Punisher, but replace the Punisher gun with a Fleshborer Hive. That's about it. Because sometimes you have just run out of Troops choices, and you just need 20 Termagants worth of Fleshborer fire on a tough platform.
 Heavy Venom Cannon
The Heavy Venom Cannon is just that: a bigger version of the Venom Cannon able to deal with bigger vehicles, and it would be a lot more reliable if it wasn't using a Blast template. Can be used against tanks up to Land Raiders, but lack the firepower to deal reliably with them and lacks the armor penetration to deal with MEQs on a consistent basis.
 Rupture Cannon
If you want to stop a tank dead in its tracks, the Rupture Cannon is your friend. Firing two projectiles in a short succession, a tick that covers the enemy in a gooey substance, and a seed that upon impact with the goo dissolves at such speed it implodes with enough force to turn a Baneblade inside out. That's what the fluff says, anyway - it strikes at AP 4, so MEQs don't even notice it.
 Stranglethorn Cannon
An upscaled version of the Barbed Strangler, the Stranglethorn Cannon has enough power to deal with even the toughest of foes, and serves as a borderline artillery weapon.
Flying under the wings of Hive Crones, the Tentaclids are living seeker missiles who can chase after flying targets and disable them with a powerful bio-electric pulse.
Certain weapons used by the Tyranids are part of the creatures themselves, instead of being addition to the arms/back.
 Bio-electric Pulse
Built into the Trygon, a electrical charge is build up by the creature as it moves, allowing a powerful and large discharge. The Trygon Prime increases the range of this blast, and doubles its rate of fire.
Exclusive to Carnifexes, it is a biological equivalent of a shot from a plasma cannon, and just as deadly. However, since Monstrous Creatures can only fire 2 of their ranged weapons per Shooting Phase, and you should already have two sets of twin-linked brainleech devourers on your 'fex, AND it's only 12-inch range, you'll never get the chance to use this biomorph and shouldn't waste the points to begin with.
 Bio-plasmic Cannon
Did we say "exclusive to Carnifexes"? Well, it still is, because this is a different version that's actually good. Mounted on the back of an Exocrine (or the "Giant Plasmavore" as I've taken to calling it) it delivers the same S7 AP2 as any other plasma, and can be fired either in Large Blast or Assault 6 modes so you can season to taste. 24-inch range.
 Cluster Spines
The largest Tyranids have banks of spines build into their carapaces that they can fire when under stress. These hollow spines explode upon impact, dealing significant damage to anything in the area of impact.
 Drool Cannon
The Hive Crone's answer to the Heldrake Baleflamer, except lacking Torrent and a good AP value and pretty much anything good, including its icky name. Template S6 AP4.
 Flesh Hooks
Until 5th Edition edition came out, nearly any species of Gaunt, Warrior and more could use these. They are long bony spines with barbed hooks on the end that serve as both harpoons and possible grapnel for climbing. Now they're Lictor and Warrior/Shrike only (Carnifexes have access to Spine Banks, which are similar), count as Frag Grenades, and can be fired for a surprisingly deadly shot.
 Grasping Tongue
Though technically part of an Haruspex's body, it can use its long tongue to snag an enemy, pluck it out of its unit and devour it whole.
 Ripper Tentacles
Used by Mycetic Spores, these are long-ranged tentacles able to do significant damage to medium-sized blobs thanks to their high rate of fire (strange choice of words when the weapon in question is akin to beating someone over the head with a tentacle)
 Spore Mine Cysts
Build into the underside of a Harpy, you can have it drop a Spore Mine onto something it flew over once per game as if it was making a bombing run. The same rules for the Spore Mines as stated above count here as well.
 Stinger Salvo
Works comparable to the Cluster Spines, except this is more of a barrage of large solid spikes akin to the spinefists.
 Thorax Swarm
Certain Tyranids can issue forth swarms of bugs living in their thoraxes to devour anything in short range. These swarms come in three flavors: Electroshock Grubs who electrocute anything they grab onto for a damaging attack, the Desiccator Larvae who drain the target's body of all of its moisture, and the Shreddershard Beetles who Rend their way through their targets, exploding in shards of needles.
Every Tyranid organism has at least rows of sharp teeth and deadly claws, but some go an extra mile.
 Acid Maw
To aid them in the digestion of a world's biomass, Pyrovores drool a highly corrosive acid that can dissolve pretty much anything. The downside is that they need to focus all their energy on making a successful bite attack, diminishing their potential damage output.
 Blinding Venom
Unique to Gargoyles, they can spit acid in the eyes of their enemies to blind them and allow for an easy devouring of the target. The poison is quite weak (6+ what the hell?) and it's best to just go with the regular poison attacks granted by Toxin Sacks.
Psychic swords that feed off the life force of their target, they can inflict Instant Death and tear through Space Marines with ease.
Badass Boneswords used by the Swarmlord, it can use these weapons in a barrage of blades to ward off incoming melee attacks.
 Crushing Claws
Used only by Carnifexes, Tervigons and Hive Guard, this massive pair of crab-like claws grants S+1, AP2, Armourbane and Unwieldy. Not very interesting for the Tervigon because its role is really nothing more than sitting on an objective, shitting out as many Gaunts as it can. Carnifexes on the other hand have pretty much lost their utility as linebreakers, you're better off running them as a dakkafex and letting Trygons do the heavy lifting. They are not very interesting on Hive Guard either, given their prohibitive cost and they make them swing very slow: it is best to use the Rending Claws and have them do the job.
 Lash Whip
A classic and the source of many a rule 34, they allow its user to strike much faster than they normally could, getting a blow in before the enemy can. The weapon suffers from not being effective against armored enemies, though. Provided you're not a venomthrope this is paired with a bone-sword with AP 3 which makes this combo death-incarnate for MEQ
 Rending Claws
The iconic claws of Genestealers and the origin of the Rending rule, these claws are both incredibly sharp and hard, allowing you to pick apart even a Terminator with ease.
 Scything Talons
The long, bladed weapons you see on Tyranids of all sizes, they allow a fast barrage of blows onto a target. They can pierce very light armor, but for the rest are the basic Tyranid melee weapon.
 Tail Biomorphs
Certain large Tyranids do not leave their tails unarmed: they can be armed with a variety of deadly augmentations: maces capable of punching holes in tanks, pincers that can injure the largest monsters, scythes who can cear through armor like it's nothing or stingers containing highly lethal toxins.