Users
Malaysia: Malaysian Special Operations Force units, the UTK and the VAT69 [3] 
Singapore: Singapore Police Force, Used by the Special Task Squadron and Emergency Response Team of the Police Coast Guard.
| 5.56mm, Advanced Piston Carbine | |
|---|---|
| Type | Assault rifle |
| Place of origin | United States |
| Service history | |
| In service | 2010s–present |
| Used by | See Users |
| Production history | |
| Manufacturer | Colt's Manufacturing Company |
| Specifications | |
| Mass | 7 lb (3.2 kg) empty 7.5 lb (3.4 kg) with 30 rounds |
| Length | 33 in (840 mm) (stock extended) 29.8 in (760 mm) (stock retracted) |
| length | 14.5 in (370 mm) |
The Colt Advanced Piston Carbine or Colt APC is a lightweight modular 5.56mm caliber piston-operated, magazine fed carbine with a one-piece upper receiver which is capable of firing in automatic and semi-automatic modes. The design incorporates an articulating link piston that reduces the stresses in the piston stroke by allowing for deflection and thermal expansion.
The weapon has a suppression-ready fluted barrel, which is lighter and cools better than previous M4 barrels. It is claimed to have "markedly better" accuracy. To improve reliability, Colt used an articulating link piston (ALP) which "reduces the inherent stress in the piston stroke by allowing for deflection and thermal expansion". [1] In traditional gas piston operating systems, the force of the piston striking the bolt carrier can push the bolt carrier downwards and into the wall of the buffer tube, leading to accelerated wear and even chipped metal. This is known as carrier tilt. The ALP (articulating link piston) allows the operating rod to wiggle to correct for the downward pressure on the bolt and transfers the force straight backwards in line with the bore and buffer assembly, eliminating the carrier tilt. This relieves stress on parts and helps to increase accuracy. [2]
Malaysia: Malaysian Special Operations Force units, the UTK and the VAT69 [3] 
Singapore: Singapore Police Force, Used by the Special Task Squadron and Emergency Response Team of the Police Coast Guard.
The M16 rifle is a family of military rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-round magazine.
In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock.
The Colt AR-15 is a lightweight, magazine-fed, gas-operated semi-automatic rifle. It is a semi-automatic version of the M16 rifle sold for the civilian and law enforcement markets in the United States. The AR in AR-15 stands for ArmaLite rifle, after the company that developed it in the 1950s. Colt's Manufacturing Company currently owns the AR-15 trademark, which is used exclusively for its line of semi-automatic AR-15 rifles.
The M4 carbine is a 5.56×45mm NATO, gas-operated, magazine-fed carbine developed in the United States during the 1980s. It is a shortened version of the M16A2 assault rifle.
The Colt Canada C7 is a Canadian family of military rifles, manufactured by Colt Canada, having similar design and function to the Colt M16A3.
Direct impingement is a type of gas operation for a firearm that directs gas from a fired cartridge indirectly into the bolt carrier or slide assembly to cycle the action. Firearms using direct impingement are theoretically lighter, more accurate, and less expensive than firearms using cleaner and cooler gas piston systems.
The Stoner 63 is a 5.56×45mm NATO modular weapon system. Using a variety of modular components, it can be configured as an assault rifle, carbine, top-fed light machine gun, belt-fed squad automatic weapon, or as a vehicle mounted weapon. Also known as the M63, XM22, XM23, XM207 or the Mk 23 Mod 0 machine gun, it was designed by Eugene Stoner in the early 1960s. Cadillac Gage was the primary manufacturer of the Stoner 63 during its history. The Stoner 63 saw very limited combat use by United States forces during the Vietnam War. A few were also sold to law enforcement agencies.
Eugene Morrison Stoner was an American machinist and firearms designer who is most associated with the development of the ArmaLite AR-15 rifle that was redesigned and modified by Colt's Patent Firearm Company for the United States military as the M16 rifle.
Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propellant charge.
A breechblock is the part of the firearm action that closes the breech of a breech loading weapon before or at the moment of firing. It seals the breech and contains the pressure generated by the ignited propellant. Retracting the breechblock allows the chamber to be loaded with a cartridge.
Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent case and insert a new cartridge into the chamber. Energy from the gas is harnessed through either a port in the barrel or a trap at the muzzle. This high-pressure gas impinges on a surface such as a piston head to provide motion for unlocking of the action, extraction of the spent case, ejection, cocking of the hammer or striker, chambering of a fresh cartridge, and locking of the action.
The Heckler & Koch HK416 is a gas-operated assault rifle chambered for the 5.56×45mm NATO cartridge. It is designed and manufactured by the German company Heckler & Koch.
The T65 is a rifle developed and manufactured by the Combined Logistics Command of the Republic of China Armed Forces in Taiwan. Originally patterned after the Armalite AR-18 that has a short-stroke gas system, the prototype unveiled in 1975 showed a rifle that is heavily influenced by the AR-15 family of rifles, albeit with modified iron sights, a reshaped stock, and redesigned handguards. The designated number '65' refers to the Year 65 of Republic of China (1976), the year the rifle's design was finalized. Upgraded versions were designated the Type 65K2 and Type 65K3.
The SG 550 is an assault rifle manufactured by Swiss Arms AG in Switzerland. "SG" is an abbreviation for Sturmgewehr. The rifle is based on the earlier 5.56×45mm NATO SIG SG 540.
The M6 is a series of carbines designed and manufactured by LWRC International. It is based on the M4 carbine, with which it shares 80% of its parts. The 'M' model name is not a US military designation. Like the HK416, it features a proprietary short-stroke self-regulating gas piston system and bolt carrier/carrier key design, which prevents trapped gases from contacting the bolt carrier or receiver of the weapon. The manufacturer claims that this reduces the heating and carbon fouling of the internals, simplifies field maintenance, and improves reliability.
The ADCOR A-556 is an assault rifle based on the M16/M4 series. It is produced by the Adcor Defense Company and was one of the contenders in the US Army Individual Carbine Competition to replace the M4 Carbine.
An AR-15-style rifle is any lightweight semi-automatic rifle based on or similar to the Colt AR-15 design. The original ArmaLite AR-15, its predecessor, was a scaled-down derivative of Eugene Stoner's ArmaLite AR-10 design and featured selective fire. ArmaLite sold the patent and trademarks to Colt's Manufacturing Company in 1959, resulting in the Colt AR-15, which removed the selective fire feature. After most of the patents for the Colt AR-15 expired in 1977, many firearm manufacturers began to produce copies of the Colt AR-15 under various names. While the patents are expired, Colt retained the trademark of the AR-15 name and is the sole manufacturer able to label their firearms as AR-15.
The Kel-Tec RDB is a bullpup carbine offered in 5.56×45mm NATO semi-automatic rifle, manufactured by Kel-Tec Industries of Florida. It uses a rotary bolt with a spring loaded ejector and an extractor facing down, ejecting spent shell casings downward, allowing easy ambidextrous use.
Carrier tilt is a wear issue that can arise in some gas piston-based firearm operating systems. High pressure gas pushes the gas piston back hitting the bolt carrier. This force pushes the bolt carrier down into the buffer tube wall. This can lead to increased wear, shaved and/or chipped metal. This in turn can lead to a loss of accuracy.
Tilting bolt action is a type of locking mechanism often used in self-loading firearms and, rarely, in straight-pull repeating rifles. Essentially, the design consists of a moving bolt driven by some mechanism, most often a piston with gas pressure from the gas port behind the muzzle. The bolt drops down into receiver recess and locks on bolt closing. Tilting the bolt up and down locks-unlocks in the breech. This tilting allows gas pressure in the barrel from firing the gun to lower to safe levels before the cartridge case is ejected.
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