The AR-15 is a modular weapon system, primarily manufactured by Colt and several other companies. It is the basis for the M16 series of service rifles currently in use by the U.S. military, the C7 and C8 series of service rifles currently in use by the Canadian military, and for many semi-automatic rifles currently manufactured by several companies.
History
The M16 Rifle (1966)
The M16 rifle, circa 1966
The AR-15 is a derivative of the original AR-10; it came about due to the fact that the U.S. Army was forced to revisit the idea of a more lightweight rifle that fired an intermediate cartridge.
The AR-15 was designed around this idea in 1958[3], being an AR-10 that was scaled down to take .223 Remington cartridges. The U.S. Air Force and the U.S. Army adopted AR-15 variants (as the M16), and modified the .223 Remington cartridge slightly (by switching the powder to one that would produce a higher velocity), this cartridge becoming the 5.56x45mm NATO.
Service in Vietnam
Tales of the Gun - M-16
Initial M16 rifles lacked a chrome-lined barrel and chamber, which left it prone to rust. The rifle was also touted as "self-cleaning" (though not by the manufacturer), and was not issued with cleaning kits (in reality, the rifle needs lubrication on all surfaces that involve metal-to-metal contact). This caused problems when the rifle was issued to line units in Vietnam; when fired, carbon fouling, in combination with a humid environment and lack of lubrication and cleaning, caused the weapon to foul up and malfunction repeatedly. This was further exacerbated by the switching of the powder to increase velocity in the 5.56 NATO cartridge.
This caused the government to look more closely at the M16. The blame was cast on the weapon itself rather than the fallacious self-cleaning claims made by individuals who misinterpreted what they heard from the designers and manufacturers.
Service in the Middle East
The M16, M4, and other AR-15 variants, have served with great results in the current wars in Iraq and Afghanistan, mainly due to the fact that troops have now learned to properly maintain the weapons (namely, having enough lubricant on the bolt carrier group to keep it cycling well). Many more variants have been fielded, to include the Mk 12 SPR, the Mk 18 CQBR, the M16A2 and M16A4, the M4, and even some older variants (mainly used by non-combat units).
Law Enforcement
The AR-15 is widely used by law enforcement in the United States as a patrol rifle. It was discovered to have superior terminal ballistics to the MP5 SMGs that preceded them, in a similarly sized package. A couple of incidents in which the 9mm Luger cartridge from the MP5 proved to be insufficient (mainly involving criminals using body armor) caused the large shift from SMGs (Uzi, MP5, and others) to the AR-15.
Civilian usage
Citizens in the U.S., Canada, Philippines, and some areas in Europe (particularly Norway, Finland, and Poland) use the AR-15 for a variety of purposes, including self-defense, competition, hunting, and plinking. These rifles are usually semi-automatic; select-fire rifles are legal in the U.S. so long as they are registered (though they can no longer be registered, due to a last minute amendment to the FOPA (Firearm Owner's Protection Act, of 1986)) before it was passed.
A few companies in Germany and Switzerland currently produce AR-15s, in both the standard expanding gas piston action and the short-stroke piston action (Heckler & Koch, with the HK416).
Design details
The AR-15 is a lightweight, magazine-fed, gas-operated, air-cooled, shoulder-fired rifle.
Most AR-15 models generally utilize the semi-automatic fire control group - the selector switch has options for SAFE and FIRE. These weapons are found with a modified hammer assembly, a modified disconnector, a modified trigger assembly, an unmilled shelf in the lower receiver (sometimes blocked to prevent the installation of a drop-in auto sear, particularly on rifles produced during the 1994 Federal Assault Weapons Ban in the U.S.), and they lack an auto sear. On the hammer, there is no notch for the auto sear to catch when the carrier cycles back. The selector switch allows the sear on the trigger assembly to catch the hammer as the bolt carrier cycles rearward, preventing the hammer from re-engaging the firing pin and firing another round until the trigger is pulled again.
A semi-automatic AR-15 bolt carrier. Note the rear part of the bottom of the carrier; there is not enough metal to engage an auto sear.
To further prevent conversion to full auto, some manufacturers use bolt carriers modified to prevent engagement with an auto sear, if one were to be installed; approximately 1.25 inches of steel is milled off the rear part of the carrier, preventing the auto sear from releasing the hammer to fire again.
Rifles manufactured for export or military sales have the automatic fire control group; the selector switch has options for SAFE, SEMI, and BURST (or AUTO). These weapons are found with the M16 hammer assembly, the M16 disconnector, and a trigger assembly that allows for the auto sear to function as intended in conjunction with the auto selector switch. The M16 lower receiver has a section of its shelf milled lower to accommodate the auto sear, and an additional hole (the sear hole) is drilled into the lower receiver to hold the sear in place.
Bolt carrier assembly
The bolt carrier assembly consists of the bolt, a firing pin, a retaining pin to hold the firing pin in its channel inside the bolt, a cam pin to secure the rotating bolt as it locks and unlocks when the weapon cycles, and a gas key, which is attached to the top of the bolt carrier itself via two Grade 8 Allen screws (hex screws), properly sized so that the staking will be able to hold them in. The gas key should be properly staked, deforming the metal enough to prevent the screws from backing out during operation of the weapon. The screws should not be secured via Loc-tite alone; heat will break down the loc-tite and the screws will back out with continued operation, breaking the seal between the gas key and the carrier, which will cause the rifle to short-stroke or even not cycle at all (as the action is not getting enough gas). The bolt is a rotating bolt design, and has seven lugs that lock into the barrel extension, sealing the chamber when it goes into battery. The AR-15 does not feature a primary extraction movement during operation.
The bolt should be composed of Carpenter 158 (C158) steel for maximum durability, and should be shot peened, tested with a proof load, and MPI tested for cracks and inclusions.[4]
Barrel and bore
In adherence to the TDP (Technical Data Package), the barrel should be hammer forged from 4150 CMV (Chrome-moly Vanadium) steel. This allows the barrel to last a bit longer under the stresses of full auto fire or rapid semi-auto fire, and leaves the barrel less prone to gas port erosion and throat erosion. Most AR-15 manufacturers tend to use the 1:9 rifling twist in their barrels, which is usually able to handle projectiles up to around 69 grains; stabilization problems sometimes result if the bullet is any heavier (because they are generally longer and require a faster twist rate). The data package calls for a 1:7 twist in the M16s and M4s, which handles the heavier and longer bullets. As with the bolt, the barrel should be tested with a proof load and MPI tested to check for structural deficiencies.
Sights
The iron sights on the A1-style AR15s did not have distance adjustment on the rear sight, only windage. The A2-style sights, as well as many backup iron sights commonly found on many modern ARs, have distance settings and windage settings; both are adjustable. The standard distance settings are 300 to 800 meters.
The A2 sights feature a smaller aperture for longer distance shooting, and a larger aperture (labeled as 0-2, for 0-200 meters) for low-light shooting and close-quarters combat.[5] Typically, if the smaller aperture is zeroed for 300 meters, the larger aperture is zeroed for 200 meters.
The standard front sight assembly on all AR-15s is adjustable for elevation.
The standard A2 rifle front sight post is .270 inches tall, and has a square profile. It has four notches around the base of the front sight post, each notch representing 1.25 MOA in elevation.[6] The standard A1 rifle front sight post is .260 inches tall, and has a tapered round shape. It has five notches around the base of the front sight post, each notch representing 1 MOA of elevation.[7]
The standard A2 carbine front sight post is .270 inches tall, and has a tapered, square profile, to provide a better view of the target. This is due to the fact that the carbine has a shorter sight radius than the rifle; the front sight post appears to cover much more of the target.
Colt makes a front sight post that is .300 inches in height, and Bushmaster makes one that is .310 inches tall, allowing for closer zeroing without the front sight being too far out of the front sight base.[8]
Operating system
The standard AR-15 utilizes what is commonly referred to as the direct impingement gas operation system. This is untrue, as the gas does not impinge directly on the bolt face itself. This system is actually a gas piston system in its own right; unlike a short-stroke or long-stroke gas piston system, there is no actuator rod to push the bolt carrier group. The bolt is the piston (more specifically, the rear part, with the gas rings (piston rings)), and the system operates on the force of expanding gas in the chamber of the bolt carrier.[9] Unlike most other gas piston mechanisms, this is an inline piston system, as opposed to short-stroke and long-stroke piston systems, which are offset piston systems.
The AR features a rotating bolt breech mechanism.
The magazine is loaded into the weapon and the charging handle is racked, pulling the bolt carrier assembly into the receiver extension. The action spring forces the bolt carrier forward, stripping a round out of the magazine and into the chamber. The bolt rotates and locks as it goes into the breech. When the trigger is pulled, the hammer strikes the firing pin, which fires the round. The gas propels the bullet toward the muzzle end of the barrel. As the bullet travels down the bore, the gas produced by the ignition of the propellant fills the gas port (a hole drilled in the barrel that leads into the port in the front sight block, which is connected to the gas tube; a common feature in all gas piston weapons) and the gas tube, which leads back into the receiver. As the gas is forced into the gas key on the bolt carrier (which is hollow all the way through) via the pressure at the gas port, the expanding gas fills the hollowed-out chamber in the bolt carrier itself until the pressure is great enough to force a rearward motion. This separates the carrier's gas key from the gas tube and cuts off the gas flow. As this happens, the bolt rotates and unlocks, and allows the weapon to cycle. There are several gas systems of differing length, as described below.[10]
The receiver extension
The receiver extension is the part that allows the cyclic motion of the bolt carrier. It contains a spring and a buffer, filled with weights, to send the bolt carrier forward once it has completed its rearward travel. The extension is threaded into the lower receiver, and helps to retain a small spring-loaded detent that retains the action spring. It is locked in place by a screw in the end of the stock (or a stainless steel nut directly on the endplate with carbines) and endplate, which also holds a spring and detent in place to retain one of the push pins that hold the lower and upper receiver together. On carbines, the endplate is staked at a point where it engages the stainless steel nut, to prevent the nut backing out during operation.
NOTE: Some AR-15 manufacturers use Loctite threadlocker to hold the nut in place. This is unnecessary, and can damage the receiver extension.
There are two different lengths for the receiver extension: carbine and rifle. Rifle receiver extensions are used with fixed-stock AR-15 rifles, and are slightly longer than the carbine-length receiver extensions. The corresponding action spring is generally between 11.75 and 13.5 inches long; any shorter, and it should be replaced.
Carbine-length receiver extensions are slightly shorter, and utilize an extra-strength action spring that is between 10.0625 and 11.25 inches long. There are several buffers that can be used with a carbine, all with different weights. This is mainly to combat the effects of gas port erosion (or just the effects of too much gas coming into the action, which causes fast unlocking, which can be a problem with some carbines. This is particularly true for the shorter gas systems). There are two different carbine receiver extensions, identical in length, but one (the so-called "commercial" variant) is .03 inches wider (1.17 inches) than the mil-spec variant, which is 1.14 inches. This can cause stocks which fit the commercial receiver extension to not fit a mil-spec diameter receiver extension.
Rifle buffers
Rifle-length receiver extensions use a buffer that is unique to it - it is longer than buffers used in a carbine system. The buffer itself has five steel weights and one spacer inside, and weighs in at 5.1 ounces. It has a reciprocal weight of 3.5 ounces (how much force it uses to push the carrier forward during a cycle).[11]
Carbine buffers
A regular carbine buffer has three steel weights. It weighs 3 ounces and has a reciprocal weight of 1.9 ounces.[12] The buffer is generally unmarked.
An "H" buffer (Heavy buffer, H1 buffer) is a carbine buffer with one tungsten weight and two steel weights. It is slightly heavier (hence the name), at 3.7 ounces, and has a reciprocal weight of 2.8 ounces. This buffer is marked with an "H".
An "H2" buffer is even heavier, at 4.6 ounces and a 3.5 ounce reciprocal weight. It has two tungsten weights and one steel weight. This buffer is generally used when a suppressor is mounted to an AR-15, or when a particular carbine is known to be overgassed. This buffer is marked with "H2".
An "H3" buffer has three tungsten weights. This buffer is generally used if the H2 buffer is not enough to slow down the cyclic rate, particularly with suppressors. This buffer is marked with "H3".
All carbine buffers have three spacers inside, along with the three weights.
Gas systems
There are, at present, five different lengths of gas system for the expanding gas AR-15. Other AR-15s generally use gas systems that are equivalent to the carbine-length gas.
Pistol-length gas system
This system is found on barrels which are too short to accommodate the carbine gas system (usually 7.5 inches). The gas port is drilled 5 inches from the chamber. Pressures in the chamber tend to run extremely high and are very hard on internal parts.
Carbine-length gas system
This system is found on barrels ranging in length from 10.3 inches to 16.5 inches. The gas port is drilled 7.5 inches from the chamber, and the gas pressure typically clocks in at around 26,000 PSI at the port. If there is less barrel after the gas port, there will be less dwell time. Because of this, the gas port on barrels 12.5 inches and shorter are drilled a little wider than it would be on a 14.5 or a 16 inch barrel to compensate for the gas system having less time charged with the pressure necessary to fully cycle the weapon. Due to the high pressures under which this system operates, these weapons are typically harder on internal parts compared to the other gas systems.
Mid-length gas system
This gas system is found on either 14.5-inch barrels or 16-inch barrels. The gas port is drilled 9.5 inches from the chamber. As a result, the gas pressure at the port is noticeably less than what would be found on a carbine-length system. The width of the gas port varies from manufacturer to manufacturer; being relatively new, there is no set standard or specification for the diameter of the port.
Intermediate-length gas system
This gas system is found on barrels with a length of at least 16 inches, and optimally at 18 inches. The gas port is drilled 11.5 inches from the chamber. This system runs similarly to the rifle-length gas system, with pressures being slightly higher. This system is found in Knight's Armament (KAC) SR-15 rifles, as well as some Noveske AR-15 rifles (they, along with the company VLTOR, developed their own intermediate-length system, which measures out to being fractions of an inch shorter than the KAC version). [13]
Rifle-length gas system
This gas system is typically found on barrels over 17 inches long. The gas port is drilled 13 inches from the chamber. As with the mid-length gas system, the gas pressure is noticeably decreased, clocking in at around 13,500 PSI. The gas port is typically drilled to a diameter of around .090 to .100 inches on these barrels to compensate for the lower pressure, thus allowing for the weapon to cycle reliably.
Ammunition and feeding system
This weapon fires both the .223 Remington cartridge and the 5.56 NATO cartridge, depending on the leade in the chamber (if the leade is longer to accommodate the higher pressures generated by the 5.56 NATO cartridge, the rifle will fire both; otherwise, it will only fire .223 Remington ammunition reliably. Chamber dimensions tend to vary among manufacturers; generally, the more commercial manufacturers' rifles tend to have tight chambers.). It fires from a detachable magazine, usually with a capacity of 30 rounds.
Manual of arms
The AR-15 and its variants are lightweight, gas-operated, magazine-fed, air-cooled, shoulder-fired weapons. They feed from box magazines, most of which are either the 20-round or 30-round variety (each variety, save for the 5-round magazines, should be downloaded by two rounds - 28 for 30, 18 for 20, etc. This makes it easier to load on a closed bolt).
AR-15s feature a bolt catch mechanism that holds the bolt open upon firing the last round in the magazine.
The magazine is inserted straight into the magazine well, and firmly pushed until a "click" is heard. Give the magazine a slight tug to make sure it does not come out. If the bolt is locked open, simply press the serrated "paddle" portion of the bolt catch in with the thumb to close the bolt and chamber a cartridge. If the bolt is closed, pull the charging handle to the rear and let it slingshot forward. The forward assist should not be needed unless the weapon isn't properly lubricated and is dirty.
At this point, turn the selector switch to the mode of fire of choice (semi (fire on some models), burst/auto), and fire the weapon.
Once the magazine is empty, the bolt will lock open. It is good practice to always check the chamber for any obstructions prior to reloading.
Maintenance
This section deals with general maintenance of AR-15 rifles and carbines.
Parts life
The numbers given below are for AR-15 CARBINES, firing 5.56 NATO ammunition; mid-length carbines and rifles have longer parts life, and parts life will be longer if shooting with .223 Remington ammunition. If the rifle in question is used a lot suppressed, or there is a lot of full auto or rapid semi-automatic fire, then the figures should be cut in half.
At 10,000 rounds, these parts should be replaced:
- Disconnector
- Extractor
- Extractor Spring and Insert
- All springs in the fire control group (Hammer spring, sear spring, disconnector spring)
- Firing pin
At 15,000 rounds, these parts should be replaced:
- Bolt
- Action spring
At 20,000 rounds, these parts should be replaced:
- Barrel
- Gas tube
Lubrication
All AR-15s, regardless of what gas system they use, operate best with heavy lubrication on the bolt carrier and the bolt. The lubrication used must not be too viscous, or it will burn off during operation.
Heavy lubrication on the bolt increases the life of the gas rings by reducing the metal-to-metal contact.
All points of contact within the receiver that involve metal-to-metal contact should be lubricated.
Lubrication is useful on the action spring and the charging handle as well; it reduces wear and noise.
Avoid lubricating the firing pin channel, especially in colder climates. This can cause the firing pin to stick.
Cleaning
Most of the time, simply wiping the dirty oil off of the parts and applying new oil is sufficient. A deep cleaning may be occasionally needed; degrease the inside of the weapon, dry it off, and then apply new lubrication.
It is not necessary to run a bore brush through the barrel more than three or four times. The barrel crown can be damaged if this is done excessively, which will adversely affect the weapon's precision.
A chamber brush should be used to clean the chamber. Use the chamber brush with a cleaning patch wrapped around it after loosening up the carbon, and wipe the chamber until it is clean. A very light coat of oil can be used here, but it is not necessary unless shooting steel-cased ammunition.
For storage, a light coat of oil on all steel parts of the weapon is beneficial. The oil should have good protectant properties.
Configurations
The rifle comes in many different configurations. The stock is either fixed (the A2 stock is 5/8ths of an inch longer than the A1 stock), or collapsible; there are many different styles of collapsible stocks produced by various manufacturers. The barrel comes in many different lengths, anywhere from 7.5 inches to 24 inches. The length of the barrel tends to dictate the length of the gas system, and the longer the gas system, the easier it is on the more critical parts (such as the bolt).
Other variants deviate from the standard expanding gas inline piston design, and use either a short-stroke or long-stroke gas piston design (which make use of actuator rods - there is no such mechanism in a standard AR-15, as the gas actuates the system through expansion). However, these tend to be proprietary; there is no set standard, and parts between these models are not usually interchangeable.
Short-stroke piston AR-15 rifles and carbines have been shown to perform better when subjected to a lot of full auto fire, as well as suppressor usage and on AR-15 rifles with shorter barrels (due to gas port sizing, though this has now been rectified with extensive research and development by some manufacturers).[14]
U.S. Military Variants
M16
First model of the M16, adopted in 1960. Featured a 20-inch barrel with a 1/12 twist and a non-chrome lined stainless steel bore, an A1 stock, fixed iron sights with rear sights adjustable only for windage, and no forward assist. Early models did not feature a brass deflector. The lower receiver did not feature a rib around the magazine release button. Featured a three-prong flash suppressor and an FCG with three functions: SAFE, SEMI, and AUTO.
M16A1
First model officially adopted by the U.S. Army. Featured a 20-inch barrel with 1/12 twist, and a chrome-lined bore and chamber. Forward assist included with this model, bird cage flash suppressor added and an external rib around the magazine release. Otherwise identical to M16. It was adopted by the U.S. Army in 1963 and was in use during the Vietnam war throughout the U.S. military, phasing out the M14.
M16A2
M16A2
Model adopted by the U.S. military around 1986. Features the A2-style lower with longer buttstock, the round delta ring, round handguards, the heavy "government profile" 20-inch barrel with 1/7 twist, the A2 pistol grip, a fire control group with SAFE, SEMI, and BURST functions, a longer stock, brass deflector and iron sights, with the rear sight being adjustable for windage and range (up to 800 meters). The front sight, as noted earlier, still adjusts for elevation.
M16A3
Same as M16A2, but with automatic fire instead of burst fire. Can also feature a flat-top upper receiver. In use with the U.S. Navy.
M16A4
Introduced in 1996, it is the same as M16A2, but with a flat-top upper receiver suitable for mounting optics. Features a rifle-length KAC (Knight's Armament Company) rail (RIS rail) and a slightly taller front sight block (if the gas block isn't shaved down for optics). This is the current issue rifle for the U.S. Marine Corps.
M4
Introduced in 1994, this model features a collapsible stock, a carbine-length gas system, shorter handguards, and a 14.5 inch barrel with a notch to accommodate the M203 underbarrel mounted grenade launcher. The barrel assembly features M4 feed ramps; whereas the feed ramps in a rifle are angled at 45 degrees, the M4 feed ramps are longer and slightly steeper (52 degrees) to enhance reliability in the carbine system. The front sight block is also slightly taller ("F" marked). The FCG has three positions: SAFE, SEMI, and BURST. The earliest models featured an A2 upper receiver; flat-top models have replaced them.
M4A1
Same as the M4, except for the FCG, which has AUTO parts in place of the BURST FCG. The M4A1 also has the heavier SOCOM profile barrel, as opposed to the Government profile barrel used on the M4.
CAR-15
Also known as the Colt Commando, XM177, or Model 733. The CAR-15 is an AR-15 carbine that features a collapsible stock, A1-style upper receiver and sights, a thin, pencil barrel, which was initially 10.5 inches long (later variants had this lengthened to 11.5 inches), a carbine-length gas system, and a FCG marked for SAFE, SEMI, and AUTO. Initially developed for U.S. special operations forces in Vietnam, circa 1965.
Later models began incorporating the AR-15's burst fire FCG instead of the AUTO, as well as the A2 sights, and even later, flat-top receivers.
Mk 12
Mk 12 SPR
United States Special Operations Forces DMR-type rifle, also used by the U.S. Marine Corps. May feature either an A1 or A2 fixed stock or a collapsible stock, a flat-top upper receiver, an 18-inch barrel with a stainless steel bore, and a telescopic sight. This rifle also features a Knight's Armament Company 2-stage trigger, and back-up iron sights. This weapon is most often used with Mk 262 ammunition, which is a 5.56 NATO load tailored for greater accuracy and better ballistics.
Mk 18 Mod 0
Mk 18 CQBR
The Mk 18 (also known as the CQBR - Close Quarters Battle Receiver) is an AR-15, intended for close-quarters combat. It features a 10.3-inch Colt barrel, Daniel Defense RIS handrail, and is usually fitted with an optic and back-up iron sights.
Canadian Military Variants
C7
The C7 is the current rifle in service with the Canadian military, manufactured by Diemaco (now Colt Canada). It features a full auto fire control group, rifle-length gas system, A1 sights, an A2 stock, A2 handguards, and a 20-inch cold hammer forged barrel. The C7A1 variant does not feature A1 sights, but an optic, manufactured by Elcan. Of note is that the rail on this weapon's upper receiver is not a MIL-STD-1913 rail, and does not preclude the use of an "F" marked front sight block due to the fact that the rail does not sit as high.
C8
A member of the 7th NL SBS armed with a Diemaco C8A1 carbine with Elcan C79 sight and Heckler & Koch AG-C grenade launcher
The C8 carbine is the carbine member of the C7 family, and is mechanically and aesthetically very similar to the Colt 653. Colt made the initial C8s for Canadian Forces as the Colt Model 725. The C8 has a 14.5-inch (368 mm) A1 profile barrel like the Colt Model 653 M16A1 carbine, but with a 1/7 rifling twist appropriate for the 5.56x45mm NATO C77 cartridge. The C8 incorporates the design improvements featured on C7 rifles.
Trivia
- The AR-15 and its variants features prominently in many video games, to include Goldeneye, Perfect Dark, Perfect Dark Zero, the Grand Theft Auto series, among others.
Gallery
References
- ↑ Official Astra Arms Website
- ↑ United States Marine Corps M16A2 manual
- ↑ The AR-15 was designed to fit this exploration into a smaller caliber around 1958, which led to the development of the 5.56x45, commercially known as the .223Rem.
- ↑ http://vpp.net.ua/10724-15-ar-colt.html
- ↑ Credits to M4Carbine.net member Molon for this information
- ↑ The sight post with the part number SP64507 has a short, square and straight configuration. This post has a nominal height of 0.270” - Molon
- ↑ The sight post associated with the M16/M16A1 series of rifles has a round shape and is also short and tapered. The part number for this post is SP61706. It is slightly shorter than the two posts described above with a height of 0.260”. - Molon
- ↑ The last sight post is Colt’s tall sight post which has a nominal height of 0.300”. It has a square and straight configuration. Its part number is SP62447. - Molon
- ↑ AR-15 patent
- ↑ http://vpp.net.ua/10724-15-ar-colt.html
- ↑ Picture of all buffers and receiver extensions, along with some stocks
- ↑ Picture of all buffers and receiver extensions. Weight and reciprocal weight is shown here.
- ↑ http://www.m4carbine.net/showpost.php?p=1336690&postcount=12
- ↑ Larry Vickers on short-stroke piston ARs