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Colt AR-15 Identification Guide
This page is intended as a guide to identify the various Colt manufactured M16 variants and derivatives. This is not a complete list of all variants produced by Colt, and only contains those variants that have appeared in media. Due to the large number of companies that make AR-15 pattern rifles that are nearly indistinguishable from the original variants, all rifles should be identified using Colt terminology in the absences of any information to the contrary, such as unique parts or visible receiver markings. The preferred term for identification, be it the model name or simply the model number, is highlighted in bold.
Glossary
Upper Receivers
Slickside
The earliest version of the AR-15 upper receiver which can easily be identified by its lack of forward assist and brass deflector. This receiver was used on the original M16, USAF weapons, and the SP1 civilian rifle.
A1
The first variant to be fitted with a a forward assist (which has a tear-drop shaped plunger head as opposed to the round plunger used on later rifles). This is the only upper to feature a forward assist without brass deflector.
A1 Flat Top
A unique upper receiver used only on the Model 656 Special Low Profile sniper rifle. It features an integral Weaver scope rail and lower profile folding backup rear sight.
A1E1
An intermediate receiver that was an A1 receiver with the addition of a brass deflector behind the ejection port. It can be distinguished from the A2 receiver by its simpler rear sight. This upper was the one that was chosen by Diemaco (now Colt Canada) for the C7 rifle and C8 carbine. It was also commonly used on the Model 723 and 733 carbines.
A2
This upper was first implemented on the M16A2. It notably differs from its predecessor, the A1 upper, by the addition of a brass deflector and a more complicated range adjustable rear sight. Also replaced the A1's 'teardrop' forward assist button with a round one.
Flat Top
Essentially an A2 upper with the fixed carry handle replaced with a MIL-STD-1913 scope rail. It can often be seen fitted with a carry handle of the same design as the A2 upper. This is the upper that is used on the majority of current military, law enforcement and civilian rifles.
A2 LMG
A unique upper receiver fitted only to the Model 750. Externally it appears as an A2 upper with the forward assist omitted (being an open bolt weapon, a forward assist would be redundant).
Flat Top LMG
A unique upper receiver fitted only to the Model 950. Externally it appears as a flat top upper with the forward assist omitted (being an open bolt weapon, a forward assist would be redundant).
SMG
The upper receiver used on the 9x19mm submachine guns and carbines is actually a modified slickside upper (a forward assist is not necessary due to the simpler and more reliable blowback action), fitted with a distinctive semi-circular brass deflector over the rear of the ejection port (which is pinned to the receiver and removable, as opposed to the integrally forged one found on 5.56x45mm rifles) and a correspondingly shortened dust cover. The reason for this enlarged deflector is due to the fact that as the SMG is a straight blowback weapons a relatively large amount of gas blasts out of the ejection port, which would be uncomfortable for a left handed shooter if not redirected.
Flat Top SMG
A flat top variant of the original SMG upper with the same removable brass deflector, this is the upper which is used on the current range of submachine guns and carbines.
Slickside (Sightless)
A unique upper receiver found only on the M231 firing port weapon. It is identical to the slickside upper used on the original M16 but features no rear sight, being designed to be aimed using tracer fire. It does however retain the carry handle that the rear sight is normally mounted in along with the windage direction arrows for adjusting the absent sight, due to it being made from a regular slickside forging without the final machining for the rear sight.
Lower Receivers
Slabside
Used only on the AR-15 prototypes and the original M16, the slabside lower can be identified by the lack of any ridges on the right side of the weapon. The push pin as the front of the lower receiver either has a flat round head, or a round head with a circular divot in the centre.
Slabside (Screw Pin)
A variant of the slabside receiver with a screw pin at the front of the receiver is used on early Sporter models up to the AR-15A2 Sporter II. This variant of the slabside lower receiver is much more common than the genuine military slabside lower, meaning it is often used as a stand in for the correct M16 lower receiver in television and movies.
Partial Fence
Most notably used on the XM16E1, although also used on a number of contemporary carbines and experimental weapons, the partial fence lower features a horizontal ridge under the ejection port, a captive push pin at the front pivot, and an unfenced magazine release button. The term partial fence is a misnomer as the ridge on the side of the receiver is unrelated to the magazine release button, but is instead a housing for the detent spring which retains the new push pin. The partial fence name has only been applied retroactively in relation to the later full fence lower.
Full Fence
The full fence lower is the standard receiver used on all AR-15s since the M16A1. It can be identified by the raised fence around the magazine release button on the right side of the weapon which is designed to lessen the chances of accidentally releasing the magazine. The design of this lower was upgraded with the adoption of the M16A2 featuring strengthening around the front and rear receiver push pins and fire selector marking on the right side of the receiver. However, due to the difficulty in identifying them from a distance and the fact that different manufacturers use slightly different designs (such as strengthening around only the front push pin or omitting the selector markings on the right side of the weapon), these have been treated as one variant for the purpose of identification. The full fence lower (both A1 and A2) is also the receiver used on Colt's 9mm SMG. For this purpose it is modified with several holes drilled in the side, which are used to pin spacers in place to hold the smaller 9mm magazine in the standard 5.56mm magazine well.
Stocks
Solid Fixed
Original Colt rifles had a solid stock without an internal storage compartment and one piece butt pad. This stock can also be identified by hinged rear sling loop. This stock was used on the original M16 and early models of the M16A1 until 1971 when it was replaced by the A1 type stock.
A1 Fixed
An improved stock designed for the M16A1, this introduced an internal storage compartment into the stock accessed by a textured trapdoor in the butt pad. The original hinged rear sling loop was also replaced by a simpler and more robust fixed loop. Despite being commonly known as the A1 stock it was not designed until 1971 (the M16A1 was released in 1967) so early model M16A1s retained the earlier solid stock with hinged sling loop.
A2 Fixed
The M16A2 used a new stock that was slightly longer (16mm, nearly indistinguishable to the regular observer) and made from a much stronger material. The easiest way to differentiate this stock from that found on the M16A1 is the squarer edges on the butt pad, and the fact that the butt pad is chequered over its entire surface (as opposed to the A1 stock which was only textured on the trapdoor cover).
1st Generation Collapsible
This stock resembles a miniature fixed stock, but can be extended by operating a lever inside the butt pad. The only weapon it was used on was the Model 607 Car-15 SMG, and was abandoned for a simpler, cheaper and more robust design for all future carbines.
2nd Generation Collapsible
A stock made from aluminium with a black polymer coating was used on the majority of carbines from the Vietnam war until 1985. Position adjustment is accomplished by depressing a lever on the underside of the stock, releasing it once the stock is adjusted to the desired length. It also has two cutouts ahead of the butt plate above and below the buffer tube to allow a sling to be wrapped around the stock in lieu of a standard sling loop.
3rd Generation Collapsible
This stock was introduced in 1985 and is commonly known as the "fiberlite stock". The shape is similar to the early coated aluminium stock but was made from plastic, and can be differentiated by its matt finish (the coated aluminium stocks had a shiny black finish) and the two vertical strengthening ribs on each side. Early models were only two position stocks, but later models were four position. This stock fitted with a thick rubber butt pad is the model which is fitted to Diemaco/Colt Canada weapons, and a version of this modified to be a six position stock is used by the Israeli Defence Force on their carbines.
4th Generation Collapsible
Often called the "M4 stock" or "6 position" stock, this was developed as a strengthened variant for use on the M4/M4A1 carbines. It features a number of ridges and recesses on the sides of the stock, a sloped butt plate and the same bottom mounted sling loop found on M16A1/A2 fixed stocks.
Wire Collapsible
Developed for emergency dismounted use on the M231 firing port weapon, this wire stock is similar to the one found on the M3 / M3A1 "Grease Gun". Although designed for the M231, it can be fitted to any AR-15 pattern rifle.
Flash Hiders
Early Duckbill
The original AR-15 flash hider, referred to as the “Duckbill”, was only seen on early prototypes. It was quickly abandoned due to the tendency of the front prongs to break off.
Duckbill
A strengthened variant of the early Duckbill flash hider, this could be seen on the majority of colt weapons prior to the M16A1.
A1
The iconic M16 flash hider often referred to as the “Birdcage”, and was on a closed design to address problems encountered with the duckbill type flash hiders snagging on vegetation. Introduced on the M16A1, it has 6 ports equally distributed around the flash hider.
A2
First developed for the M16A2, this flash hider is similar in appearance to the original birdcage except it features five ports spread only over the top half of the flash hider. Although often described as a compensator this was not the intended purpose of the modification, but was instead designed to reduce the amount of material that would be kicked up by the muzzle blast when firing in the prone position.
3.5" Moderator
Resembling a small sound suppressor, the 3.5” moderator was designed for the Model 607 (which originally used a duckbill flash hider) in an attempt to mitigate the large muzzle flash and blast created when using such a short barrel. It featured a number of internal baffles to reduce not only the flash but also the sound report, meaning it is classed as a suppressor making civilian use more legally problematic (due to this the majority of moderators seen in film and television are replicas omitting the internal baffles and chambers). Although it succeeded in reducing the flash somewhat it led to excessive fouling in the chamber, along with accuracy problems using tracers.
4.25" Moderator
An improved version of the 3.5” moderator, this features a slotted expansion chamber at the end further reducing the weapon report. The majority (although not all) of the moderators made featured a narrow section at the back with a rib behind it referred to as a “grenade ring” to allow mounting of an under-barrel M203.
Target Crown
During the Assault Weapons Ban Colt produced a line of weapons known as "Match Target" rifles which did not feature a threaded barrel for a flash hider, instead having the barrel end with a plain target crown.
Bayonet Lug
The bayonet lug on AR-15 rifles is a small protrusion on the front underside of the gas block. This is present on the majority of Colt AR-15 military rifles, but is omitted in the case of some carbines as it is redundant due to the bayonet requiring a longer barrel to be mounted onto. It is also absent from a number of civilian rifles, most notably the Match Target rifles, in order to comply with various laws and regulations.
