A tranquillizer gun (also spelled tranquilizer gun or tranquilliser gun), capture gun, or dart gun, is a non-lethal gun used for capture via a special chemical. Tranquillizer guns shoot darts filled with tranquillizer that, when injected, temporarily sedates an animal or human, so that it may be handled (or captured) safely. The tranquillizer can be a sedative, anaesthetic, or paralytic agent. Tranquillizer guns have a long history of use for capturing wildlife without injury. Tranquillizer darts can also be fired by crossbow or breath-powered blowgun.
While for thousands of years various tribal peoples have used poisoned arrows, (for example tipped with Curare), to incapacitate animals before killing them, the modern tranquillizer gun was invented only in the 1950s by New Zealander Colin Murdoch. While working with colleagues who were studying introduced wild goat and deer populations in New Zealand, Murdoch had the idea that the animals would be much easier to catch, examine, and release if a dose of tranquillizer could be administered by projection from afar. Murdoch went on to develop a range of rifles, darts, and pistols that have had an enormous impact on the treatment and study of animals around the world.
In Kenya in the early 1960s, a team headed by Dr. Tony Pooley and Dr. Toni Harthoorn discovered that various species, despite being of roughly equal size (for example, the rhinoceros and the buffalo), needed very different doses and spectra of drugs to safely immobilize them.
The dart, usually .50 caliber (12.95 mm), is essentially a ballistic syringe loaded with an immobilizing drug and hypodermic needle, is propelled from the gun by means of compressed gas. In flight, the dart is stabilized by a tailpiece, a tuft of fibrous material, making it behave somewhat like a badminton shuttlecock. The same syringe design may be used interchangeably in certain blowguns. The needle may be plain, or collared; a collared needle has a barb-like circumferential ring that improves retention of the needle and syringe for recovery and to assure that the full dose is administered.
On impact with the animal, the momentum of a steel ball at the rear of the dart pushes the syringe plunger and injects the dose of barbiturate or other drug into the animal. The drug causes torpor and prostration within minutes. Because of the power of the drugs, the handlers then have to move quickly to secure the animal for transport, monitor its vital signs, protect its eyes and ears, and then inject antidotes when needed. Many large animals are acutely sensitive to stress and can easily die without careful treatment; in order to counter stress in targeted animals, the gun is quiet, and there is usually a valve on the gun to control the dart velocity.
These substances have been invented for animal injection only. Others such as fentanyl and haloperidol have legitimate human medical use. The effects on humans may vary in relation to specific animals- a common and potentially lethal effect of most of these drugs is hypoventilation. The injection or consumption of only a drop of M–99 is sufficient to kill an adult man within a few minutes if the correct antidote treatment is not administered immediately.[dead link] Therefore, instead of the substances found above, only incapacitating agents would be suitable for military or police use.
Military and police use
Tranquillizer darts are not generally included in military or police less-than-lethal arsenals because no drug is yet known that would be quickly and reliably effective on humans without the risks of side effects or an overdose. This means that effective use requires an estimate of the weight of the target to be able to determine how many darts (if any) can be used. Shooting too few would result in no effect whatsoever, while too many can kill the target 
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- Anaesthetics also used in dart guns
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- Tranquillizer agents
- Tranquillizer agents
- According to James Butts, Santa Monica, CA Chief of Police, "Tranquilizing agents don't affect everyone uniformly. Therefore you cannot predict whether or not you have a sufficient dose to tranquilize the individual. Second, any tranquillizer will take time to enter the bloodstream and sedate the individual. If someone is advancing on you with a deadly weapon or a threatening object, there's no way a tranquillizer would take effect in the two to three seconds it would take someone to seriously injure you." Quoted by: Buren, Abigail Van. "Could Police Use Tranquilizer Darts?." San Francisco Chronicle. (June 8, 1997): 73.