AR - 15 The AR - 15 is a lightweight , 5 . 56 mm / . 223 - caliber , magazine - fed , air cooled rifle with a rotating - lock bolt , actuated by direct impingement gas operation or long / short stroke piston operation . It has been produced in many different versions , including numerous semi - automatic and selective fire variants . It is manufactured with extensive use of aluminum alloys and synthetic materials . The AR - 15 was first built by ArmaLite as a small arms rifle for the United States armed forces . Because of financial problems , ArmaLite sold the AR - 15 design to Colt . After modifications ( most notably the relocation of the charging handle from under the carrying handle like the AR - 10 to the rear of the receiver ) , the new redesigned rifle was subsequently adopted as the M16 rifle . Colt then started selling the semi - automatic version of the M16 rifle as the Colt AR - 15 for civilian sales in 1963 and the term has been used to refer to semiautomatic - only versions of the rifle since then . Although the name " AR - 15 " remains a Colt registered trademark , variants of the firearm are independently made , modified and sold under various names by multiple manufacturers . History . The AR - 15 is based on the 7 . 62 mm AR - 10 , designed by Eugene Stoner , Robert Fremont , and L . James Sullivan of the Fairchild ArmaLite corporation . The AR - 15 was developed as a lighter , 5 . 56 mm version of the AR - 10 . The " AR " in all AR pattern rifles stands for Armalite Rifle , and can be found on many of the company ' s firearms : AR - 5 a . 22 caliber rifle , the AR - 7 , another . 22 caliber , the AR - 17 shotgun , in addition to the AR - 10 semi - automatic rifle . ArmaLite sold its rights to the AR - 10 and AR - 15 to Colt in 1959 . After a tour by Colt of the Far East , the first sale of AR - 15s was made to Malaya on September 30 , 1959 , with Colt ' s manufacture of their first 300 AR - 15s in December 1959 . Colt marketed the AR - 15 rifle to various military services around the world , including the U . S . Navy , Air Force , Army , and Marine Corps . The AR - 15 was eventually adopted by the United States military under the designation M16 . Colt continued to use the AR - 15 trademark for its semi - automatic variants ( AR - 15 , AR - 15A2 ) which were marketed to civilian and law - enforcement customers . The original AR - 15 was a very lightweight weapon , weighing less than 6 pounds with empty magazine . Later heavy - barrel versions of the civilian AR - 15 can weigh upwards of 8 . 5 lb . Today the AR - 15 and its variations are manufactured by many companies and are popular among civilian shooters and law enforcement forces around the world due to their accuracy and modularity ( for more history on the development and evolution of the AR - 15 and derivatives see M16 rifle ) . The trademark " AR15 " or " AR - 15 " is registered to Colt Industries , which maintains that the term should only be used to refer to their products . Other AR - 15 manufacturers make AR - 15 clones marketed under separate designations , although colloquially these are sometimes referred to by the term AR - 15 . Some notable features of the AR - 15 include : Semi - automatic AR - 15s for sale to civilians are internally different from the full automatic M16 , although nearly identical in external appearance . The hammer and trigger mechanisms are of a different design . The bolt carrier and internal lower receiver of semi - automatic versions are milled differently , so that the firing mechanisms are not interchangeable . This was done to satisfy United States Bureau of Alcohol , Tobacco , Firearms and Explosives ( ATF ) requirements that civilian weapons may not be easily convertible to full - automatic . In the late 1970s and early ' 80s , items such as the " Drop In Auto Sear " or " lightning - link , " conversion to full automatic was very straightforward ( sometimes requiring machining of the lower receiver with use of a mill and M16 Bolt Carrier Group ) . Such modifications , unless using registered and transferable parts made prior to May 19 , 1986 , are illegal . ( The Firearm Owners Protection Act in 1986 has redefined a machine gun to include individual components where a semi - automatic firearm can be converted to full - automatic based on a 1981 ATF ruling on machine gun parts . ) Since 1993 , The Bolt Carrier Groups used in AR - 15 type rifles for civilians have employed additional measures to prevent modification to full auto . Colt AR - 15 ' s use a metal alloy wall separating the Fire Control group from the Sear , preventing use of such items . Automatic variants have a three - position rotating selective fire switch , allowing the operator to select between three modes : safe , semi - automatic , and either automatic or three - round burst , depending on model . Civilian Colt AR - 15 models do not have three - round burst or automatic settings ; they can only be fired as a semi - automatic , and are therefore not selective fire weapons . In semi - automatic - only variants , the switch only functions to rotate between safe and fire . Some other manufacturers may mark their rifles with three - positions for collectors and re - enactors , though the guns will not fire in those modes . However weapons modified to full automatic using a lightning - link are capable of full automatic fire only — unless a special full automatic fire select mechanism and modified selector - switch is substituted . Operating mechanism . The main mechanism of operation for the rifle is known as direct gas impingement . Gas is tapped from the barrel as the bullet moves past a gas port located above the rifle ' s front sight base . The gas rushes into the port and down a gas tube , located above the barrel , which runs from the front sight base into the AR - 15 ' s upper receiver . Here , the gas tube protrudes into a " gas key " ( bolt carrier key ) which accepts the gas and funnels it into the bolt carrier . The bolt and bolt carrier together form a piston , which is caused to expand as the cavity in the bolt carrier fills with high pressure gas . The bolt is locked into the barrel extension , so this expansion forces the bolt carrier backward a short distance in line with the stock of the rifle to first unlock the bolt . As the bolt carrier moves toward the butt of the gun , the bolt cam pin , riding in a slot on the bolt carrier , forces the bolt to turn and unlock from the barrel extension . ( The gas system only serves to unlock the bolt once the projectile has exited the barrel ) . Once the bolt is fully unlocked it begins its rearward movement along with the bolt carrier . The bolt ' s rearward motion extracts the empty cartridge case from the chamber , and as soon as the neck of the case clears the barrel extension , the bolt ' s spring - loaded ejector forces it out the ejection port in the side of the upper receiver . The bolt is much heavier than the projectile , and along with the recoil - spring pressure inside the stock buffer - tube performs the cartridge ejection function and chambers the following cartridge . Behind the bolt carrier is a plastic or metal buffer which rests in line with a return spring that pushes the bolt carrier back toward the chamber . A groove machined into the upper receiver traps the cam pin and prevents it and the bolt from rotating into a closed position . The bolt ' s locking lugs then push a fresh round from the magazine which is guided by feed ramps into the chamber . As the bolt ' s locking lugs move past the barrel extension , the cam pin is allowed to twist into a pocket milled into the upper receiver . This twisting action follows the groove cut into the carrier and forces the bolt to twist and " lock " into the barrel ' s unique extension . Variants . The AR - 15 rifle is available in a wide range of configurations from a large number of manufacturers . These configurations range from short carbine - length models with features such as adjustable length stocks and optical sights , to heavy barrel models . Due to the rifle ' s modular design , one upper receiver can quickly and easily be substituted for another . There are many aftermarket upper receiver s that incorporate barrels of different weights , lengths and calibers . Some available calibers for the AR - 15 platform are the . 223 Remington / 5 . 56x45mm , . 45 ACP , 5 . 7x28mm , 6 . 5 mm Grendel , 6 . 8 mm Remington SPC , . 50 Beowulf , . 50 BMG , and . 458 SOCOM . It is not recommended to chamber the 5 . 56x45 NATO into a rifle designated . 223 Remington , due to the increased chamber pressure in the 5 . 56mm cartridges ; the two calibers are similar , but not identical . When installing a new complete upper receiver , particularly one designed to handle a different caliber of ammunition ( i . e . , other than . 223 Remington or 5 . 56x45 mm NATO ) , some modification to the contents of the lower receiver may be required , depending on the particular conversion . For example , a conversion to 9 mm typically would involve the installation of a magazine well block ( to accommodate a typical 9 mm magazine , such as Uzi or Colt SMG ) , replacing the . 223 hammer with one designed for 9 mm ammunition , and depending on the original stock , replacing the buffer , action spring and stock spacer with those designed for the new 9 mm AR - 15 configuration . The 9mm cartridge fires from an unlocked breech , or straight blow - back — rather than a locked breech , because the spring and bolt provide enough weight to allow this type of functioning . These guns do not utilize the direct gas impingement method of operation like the original . Some AR - 15 ' s like the POF , LWRCI , H & K , Sturm Ruger , Sig Sauer , and Adams arms offerings replace the DGI ( direct gas impingement ) operating system with a short stroke / long stroke gas piston system . These guns usually have modified bolt carriers , gas keys , and gas blocks . When fired , DGI systems dump high pressure hot gas through the gas tube to the bolt carrier key and into the bolt carrier group . This can rapidly heat up the bolt carrier group and cause excessive fouling , one of the main complaints about the design . Gas piston operating systems alleviate these problems , but can be the cause of other issues such as bolt carrier tilt . Some manufacturers offer upper and lower receivers machined from a solid billet ( block ) of aluminum as opposed to an aluminum forging . These include Double D Armory , LTD , Sun Devil manufacturing , LAR Grizzly manufacturing , POF - USA , and Black Rain . This is usually done for added strength . Upper receivers utilizing a monolithic rail system that combine a railed hand guard and upper receiver into one uninterrupted piece are made by companies like Colt ' s Manufacturing Company , Lewis Machine and Tool ( LMT MRP ) , POF - USA , and VLTOR . This is done to provide a continuous uninterrupted rail section that runs along the top of the gun from the weapons charging handle to the front sight / gas block . This rail section is used for the mounting of sights , laser aiming devices , night vision devices , and lighting systems . A side charging upper receiver has been developed by LAR Grizzly . The charging handle can be had in a left side , right side , or ambidextrous configuration . Since the charging handle is attached to the bolt carrier making it a reciprocating design , it can be utilized as a forward assist device as well . Early models had a 1 : 14 rate of twist for the original 55 grain ( 3 . 6 g ) bullets . This was changed to 1 : 12 when it was found that 1 : 14 was insufficient to stabilize a bullet when fired in cold weather . Most recent rifles have a 1 : 9 or 1 : 7 twist rate . There is much controversy and speculation as to how differing twist rates affect ballistics and terminal performance with varying loads , but heavier projectiles tend to perform better with faster rifling rates . Additionally , the various non . 223 / 5 . 56 calibers have their own particular twist rate , such as 1 : 10 , 1 / 11 and 1 / 12 for 6 . 8x43mm SPC , 1 / 10 7 . 62x39mm , and 1 : 12 for . 308 Winchester . Standard issue magazines are 20 - or 30 - round staggered - column magazine s , traditional box magazines exist in 40 - and 45 - round capacities , and usable magazines have been constructed from a variety of materials including steel , aluminum , and high - impact plastics . Drum magazine s with 90 - and 100 - round capacities exist , such as Beta C - Mag s . Low - capacity magazines , usually of a 5 - or 10 - round capacity , are available to comply with some areas ' legal restrictions , hunting , and because larger magazines can inhibit shooting from a benchrest . Surefire is now offering extended capacity magazines in 60 - and 100 - round capacity configurations . These magazines are a staggered column design . Muzzle devices . Most AR15 rifles have a barrel threaded in 1⁄2 - 28 " threads to incorporate the use of a muzzle device such as a flash suppressor , sound suppressor or muzzle brake . The initial design had three tines or prongs and was prone to breakage and getting entangled in vegetation . The design was later changed to close the end to avoid this and eventually on the A2 version of the rifle , the bottom port was closed to reduce muzzle climb and prevent dust from rising when the rifle was fired in the prone position . For these reasons , the US Military declared this muzzle device a compensator ; but it is more commonly known as the " GI " or " A2 " flash suppressor . Some jurisdictions have banned or severely restrict usage of flash suppressors by law . In most of these areas , AR - 15 shooters have installed muzzle brake s or compensators on their rifles . The threaded barrel allows sound suppressors with the same thread pattern to be installed directly to the barrel , however this can result in complications such as being unable to remove the suppressor from the barrel . A number of suppressor manufacturers have turned to designing " direct - connect " sound suppressors which can be installed over an existing flash suppressor as opposed to using the barrel ' s threads . Legal status of civilian ownership . Australia . AR - 15 rifles , like all semi - automatic rifles , are subject to strong restrictions of ownership in all states and territories in Australia . The only means of legally owning an AR - 15 / M16 - type rifle in Australia today beyond law enforcement is to have a Category D Firearms License ( e . g . a professional animal culler ) , to have a Firearms Collector ' s License and the firearm deactivated ( with the barrel plugged up and the action welded shut ) , or converted to blank fire if one is a member of a military re - enactment organization . The heavy restrictions on semi - automatic rifles were introduced in 1996 in response to the Port Arthur massacre – one of the firearms used in the attack was an AR - 15 . Before 1996 , AR - 15 rifles were legal to own in Queensland and Tasmania . Strict firearms laws have made the AR - 15 rifles too expensive for television and film production with the requirement to destroy or export imported semi - automatic rifles after use . Warwick Firearms & Militaria , a Melbourne prop maker , self - manufactures blank firing AR - 15 - type , " WFM4 " rifles locally with approximately three dozen having been sold . Austria . In Austria , semi - automatic centerfire rifles have to be classified as sporting or hunting firearms in order to obtain civilian - legal status . After this classification , they are considered " category B " firearms , which means that holders of gun licenses may own them . These licenses are may - issue items if the applicant specifies a valid reason ( self - defense at home for example is considered valid by law in any case ) , passes a psychological test and attends a gun - basics course . Currently , three AR - 15 manufacturers , all producing in Germany have had versions of their AR15 models successfully classified as class B weapons . These Austrian versions differ slightly from the original design in order to ensure that no military full - auto trigger , bolt and barrel may be installed . Additionally , bayonet lugs and flash hiders are prohibited on semi - automatic rifles while Muzzle brakes and compensators are legal . There is no minimum length for barrels , therefore even barrel lengths as short as 7 . 5 " are possible . Canada . The Government of Canada classifies the AR - 15 ( and its variants ) as a " restricted firearm " . For anyone wanting to lawfully own an AR - 15 , they must obtain a Possession and Acquisition License ( PAL ) valid for restricted firearms and then each acquisition of a restricted class firearm is subject to approval by the Chief Firearms Officer ( CFO ) of the would - be buyer ' s province of residence . With the introduction of strict gun control measures by former Prime Minister Kim Campbell ( Bill C - 17 ) , the AR - 15 had originally been intended to be classified as a prohibited firearm , making it all but impossible to privately own one . However , due to the presence of nationwide Service Rifle target shooting competitions , the AR - 15 was granted a sporting exception . As with all Restricted firearms ( including most pistols , some shotguns , and some rifles ) AR - 15s are allowed to be fired only at certified firing ranges since the CFOs of all provinces and territories have agreed to issue ATTs ( Authority To Transport ) for these guns only to certified ranges . Since owners can ' t legally take these guns anywhere else that shooting is allowed , they can in effect only shoot them on certain ranges . In order to legally own and transport a Restricted firearm , the firearm must be registered with the Royal Canadian Mounted Police Canadian Firearms Program and must apply for an Authorization to Transport ( or ATT ) from the Chief Firearms Officer ( CFO ) for their province or territory . Additionally , the firearm must be unloaded , deactivated by a trigger or action lock , and be in a locked , opaque container during transport . The issuance of ATTs varies considerably from province to province , and is generally reflective of a particular province ' s political and social levels of acceptance toward gun ownership . In Ontario the only way to obtain an ATT for restricted firearms is to become a member of a range , whereas in Alberta , where firearms ownership is widely accepted , generally a single ATT is promptly issued that allows citizens to transport firearms to border crossings , gunsmiths , and shooting ranges . Firearms transfers in provinces such as Quebec can take up to 3 months to process . Germany and Finland . In Germany and Finland , possession of semi - automatic rifle s , including the AR - 15 , is legal , provided that the rifle ' s owner acquires a permit for owning one . A license is required for each individual firearm and there needs to be a specific reason for ownership such as participation in the shooting sports and hunting . In Finland maximum magazine capacity in hunting is 3 rounds . But in addition you can have 1 round chambered which brings your direct ammo capacity up to 4 rounds . For German hunters , their semi - automatic firearm ' s magazine must be modified in such a way that its maximum capacity is only 2 rounds ( excluding handguns ) , meaning that when hunting game animals only 3 shots in total can be fired ( as one additional round is loaded in the chamber ) without reloading . This rule is stated in German hunting law and not in German gun law . In fact , magazines are not regulated by German gun law and free to purchase and possess in any capacity and type for anyone . France . In France , any firearms using military calibers ( 9mm , 5 . 56 NATO , 7 . 62x39 , 7 . 62 NATO , . 45 ACP , . 50 BMG , . 50 AE . ) are authorized as category 1 weapons . While fully automatic category one weapons are highly restricted , semi - automatic ones are legal for civilian possession . A hunting or sports shooting license is required to posses and purchase any firearm in France . Italy . In Italy , the AR - 15 rifle belongs to B7 class and can be owned by civilians , provided it is incapable of fully automatic fire . Like every other gun , it must be registered and to purchase it citizens must have a valid license , which is granted to every person who qualifies . Most of the rifles are chambered in . 223 , but 5 . 56x45 is also allowed . Since long arms with a caliber larger than . 22 are considered hunting weapons , it is possible to own unlimited AR - 15s as they are hunting rifles according to the law . Ireland . In Ireland , semi - automatic AR - 15s are only legal if they are rim - fire and they have their magazines blocked to 10 rounds . Any other semi - automatic AR requires a restricted license . Non semi - automatic ARs are legal up to . 308 and further caliber ARs required restricted firearms licenses . Sweden . The AR - 15 , like all other semi - automatic rifles , is legal for individuals who need one for competitive use ( IPSC rifle or 3 - gun matches ) . A valid competition license is required , and all weapons are registered with the police . There are no banned " assault weapon " features or parts . However , the AR - 15 is not allowed for hunting use . South Africa . The AR - 15 like any other semi - automatic long arm in South Africa , is legal for anyone who holds any of the following licenses : While not prohibited common citizens can only own semi - automatic AR - 15s if they are members of a hunting or target club , and possess dedicated sport person or dedicated hunter status granted by organisations accredited by the SAPS ( South African Police Service ) . Other licenses allowing the possession of semi - automatic rifles are only available to people who require their use in the conduct of their business ( e . g . security personnel ) , and citizens who can convincingly prove to the Registrar that non - restricted firearms are not sufficient to provide protection . The latter requires a specific motivation for the need of a restricted firearm for self - defence and have been granted to rhino farmers . United Kingdom . As with all semi - automatic , centerfire rifles , AR - 15s are classed as a Section 5 weapon , i . e . , a person must provide an exceptional reason and gain permission from the Home Secretary , making ownership all but impossible for a private citizen . However , AR - 15s in a manually operated straight pull configuration or semi - automatic AR - 15s that are chambered to fire a . 22 rimfire cartridge are legal and can be held on a standard Section 1 Firearms Certificate . There are no restrictions on ' assault weapon features ' in the UK , and no restrictions on magazine capacity . There are a number of UK manufacturers of " straight - pull " AR - 15 variants . Southern Gun Company has tried to introduce a 9mm " self - ejecting " variant for gallery rifle shooting nicknamed the " Unicorn " but , despite numerous units being sold on the understanding that the rifle was a compliant Section 1 firearm , the rifles were seized and subjected to stringent testing by the UK Forensic Science Service ( FSS ) . A small number of pre - production models were found to be non - compliant with section 1 status . However , later models were deemed Section 1 compliant and were returned to their owners . United States . There are no federal restrictions on the ownership of AR - 15 rifles in the United States . During the period 1994 – 2004 variants with certain features such as collapsible stocks , flash suppressor s , and bayonet lugs were prohibited for sales to civilians by the " Violent Crime Control and Law Enforcement Act of 1994 " , with the included Assault Weapons Ban . Included in this was a restriction on the pistol grip that protrudes beneath the stock , which was considered an accessory feature under the ban and was also subject to restrictions . Some rifles were manufactured with a grip not described under the Ban installed in its place . Those AR - 15s that were manufactured with those features , as well as the accompanying full capacity magazines , were stamped " Restricted Military / Government / Law Enforcement / Export Only " . The restrictions only applied to guns manufactured after the ban took effect . It was legal to own , sell , or buy any gun built before 1994 . Hundreds of thousands of pre - ban ARs were sold during the ban as well as new guns redesigned to be legal . Since the expiration of the Federal AWB in September 2004 , these features became legal in most states . Since the expiration of the ban , the manufacture and sale of then - restricted rifles has resumed completely . At least five states ( NY , NJ , CA , MD , CT ) regulate possession of AR - 15 rifles either by the restriction of certain features or outright bans of certain manufacturers ' models . For example : the A3 tactical carbine pictured above is legal for sale and possession in the United States generally , but is illegal for sale in California and the Colt AR - 15 is banned by name in New Jersey . Under U . S . firearms laws , the lower receiver of the AR - 15 is considered a firearm and subject to purchasing restrictions . ( This is not universally the case with rifles . On some other rifles , such as the FN FAL , Heckler & Koch 91 , 93 , ( G - 3 , G - 33 ) , 94 , MP - 5 or SP - 89 ( plus clones ) , the upper receiver is the serial - numbered part , and thus the firearm . ) The AR - 15 upper receiver assembly is considered a part , and may be purchased and mail - ordered in most locations . This is a desirable feature for enthusiasts , who can purchase a number of upper receivers ( often in different calibers and barrel lengths ) and interchange them with the same lower receiver . Adding a shoulder stock to an AR - 15 with a barrel shorter than 16 " would constitute constructing a Short - Barreled Rifle ( SBR ) under NFA rules – subject to a $ 200 tax stamp . The receiver , or serial - numbered part is still considered a firearm , but a receiver has unique status assigned by the Gun Control Act of 1968 as amended , and ATF regulations or rulings . ATF ruling 07 - 07 - 2009 illustrates a receiver ' s unique legal status even if the receiver can only be made into a rifle . Under the " United States v . Thompson - Center Arms Company " Supreme Court ruling , an individual can possess parts for both the rifle and pistol so long as they are not assembled improperly . This ruling has been further clarified by the ATF Director in a ruling ( ATF Ruling 2011 - 4 ) dated July 25 , 2011 which restates most of the findings in the Thompson case . Following the 1992 ruling , the ATF claimed that the finding in " United States v . Thompson - Center Arms Company " only applies to products of Thompson Contender , and not to any other companies ' products . This has changed under ATF ruling 2011 – 4 which states Furthermore , adding a forward pistol grip to an AR - 15 designated as a pistol constitutes manufacture of an AOW ( any other weapon ) . Both of these actions require an approved " Form 1 " and payment of a $ 200 tax prior to the actual construction of the item . Current wait times for approval average 5 – 8 months during which time no modifications or construction may be done . As of 2012 , there are an estimated 2 . 5 - 3 . 7 million rifles from the AR - 15 family in civilian use in the United States . They are favored for target shooting , hunting , and personal protection , and have become the most popular rifle in America . Individual states . California . The Roberti - Roos Assault Weapons Control Act of 1989 banned Colt AR - 15 rifles by name in the State of California . California ' s assault weapons ban following the Supreme Court of California ' s 2000 decision in " Kasler v . Lockyer " went further and banned AR - 15s made by other manufacturers by name . AR - 15 - style rifles that are not named specifically by the Roberti - Roos or other restricted lists can be purchased in the state with some minor modifications . Since these are not on the various lists of prohibited firearms , their lower receivers ( the part that is legally the firearm ) are referred to as " Off List Lowers " ( OLL ) . These OLLs are very common in California , and at least several hundred thousand of them have been sold in the state since the ban went into effect . Malfunction . With the plethora of manufacturers of complete weapons and aftermarket barrels , there is a potential hazard associated with chamber specifications . Both civilian ( SAAMI ) specification . 223 Remington and 5 . 56 mm NATO are available . Though the external dimensions of the two cases are the same and both chambers typically accept both types of ammunition , the firing of military specification ammunition in civilian specification chambers can produce chamber pressures greater than the barrel is designed to handle . Internally the 5 . 56x45mm case wall is thicker , and the round itself is typically loaded to produce higher pressure than the . 223 . The most common result of firing military 5 . 56x45mm ammunition in a . 223 Remington chamber is that the primer can be forced out of the case by chamber pressure , often resulting in the primer becoming lodged somewhere in the action of the rifle , and disassembly of the rifle is often necessary to remove the jammed primer . A few AR - 15 manufacturers incorporate the use of a hybrid chamber specification known as the Wylde chamber . Designed by and named after Bill Wylde of Greenup , IL , this chambering was designed to accurately shoot the military ball ammo of the day while still feeding reliably . Coincidentally , it shoots the longer 80 gr bullets commonly used in the sport of Highpower Rifle Competition very well and is one of the preferred chambers for that use . While the Wylde chamber allows for optimal seating depth of 80 grain bullets over . 223 Remington and 5 . 56 NATO , it is capable of accepting both ammunition types . The Wylde chamber is used by many manufacturers who sell " National Match " configuration AR - 15 rifle , barrels , and upper receivers . The type of chamber , manufacturer , and rifling twist in inches is typically found stamped into the barrel in front of the front sight assembly . An additional point of concern in the design is the inertial firing pin . A lightweight firing pin rides in a channel inside the bolt unrestrained . When the bolt locks forward during loading , the firing pin typically rides forward and impacts the primer of the chambered round . In military specification ammunition and quality civilian ammunition , this is not normally enough to fire the round and only leaves a small " ding " on the primer . With more sensitive primers or improperly seated primers , this can cause a slamfire during loading . Another type of malfunction , Hammer Follow , is also a potential problem for AR platform weapons . In one case , David Olofson , an Army Reservist with 16 years in the US Army , lent to a prospective buyer an AR - 15 rifle which was found to malfunction in such a way that it would fire multiple rounds on a single trigger pull . After law enforcement intervention , David Olofson was charged with and found guilty of illegally transferring a machine gun and was sentenced to thirty months in prison . Calibers . Rifle cartridges . In addition , the AR15 lower receiver can be used as a trigger mechanism for single shot or side - fed upper receiver platforms that shoot in a variety of larger calibers , including . 50 BMG and Crossbow bolts .