October 19, 1999

TECHNICAL NOTE 6: TECHNICAL INFORMATION CONCERNING AR-10B SERIES RIFLES

The purpose of this technical summary is to answer repeated questions concerning ArmaLite AR-10B series rifles, with an emphasis on differences between the AR-10B series, the M-16 series service rifle, and the SR-25 rifle produced by Knight's Armament Company (KAC).

GENERAL:  ArmaLite's priorities during development of the AR-10 were technical excellence first, and interchangeability with the KAC SR-25 and the M-16 following.  Because of the small number of early AR-10s produced between the late 1950s and early 1960s, no effort was made to maintain interchangeability with them, and no testing has yet been conducted to determine which parts interchange.

The design of the SR-25 emphasized interchangeability with the M-16 considerably more than ArmaLite has with the AR-10.  While both approaches are sound, the difference in priority has resulted in a number of technical advantages in the ArmaLite AR10 design.

For brevity, the new ArmaLite rifles are collectively referred to as AR-10s.  Four models of the AR-10B series exist:

AR-10B:  The base model of the new ArmaLite line.  It is patterned after early 1960 era rifles, and is produced largely for nostalgic reasons.  The charging handle looks like an upside down trigger and is located at the front of the carrying handle.  It special brown furniture and a rear sight elevation window at the rear of the carry handle.

AR-10A2; virtually identical to the M16A2 service rifle, but caliber .308.

AR-l0A4:  A flexible combination of the AR-10A2 and the AR-10(T).  It is generally identical to the AR-10A2 except that the upper receiver and gas block are manufactured with the “Picatinny Rail” sight mounting surfaces of the AR-10(T).  Removable iron or telescopic sights are optional.

AR-10(T); target model featuring a 24 inch long, medium weight target barrel, tubular handguard, match trigger, and Picatinny Rail receiver and gas block. 

Each rifle is matched by a similar carbine: virtually identical save for a 16 inch long barrel.  The T carbine is equipped with a standard trigger rather than the match trigger of the AR-10(T) rifle.

BUTTSTOCK:  M-16A2 type shell.  A rounded buttplate is used to reduce sharp, uncomfortable edges.  A thicker, nylon trap door is used provide protruding checkering for good grip.  ArmaLite's feature olive green furniture (stocks, grips, and handguards) as a standard feature. Black furniture is available as an option.

BUFFER:  Similar to that of the M16 rifle, but dimensioned for the AR-10.  It is of aluminum construction containing the same weights and buffer disks used in the M-16.  The KMC buffer is a simple turned nylon plug.  The purpose of the weights is to reduce elastic rebound of the carrier from the barrel extension.

OPERATING SPRING:  32 coil 17-7 stainless steel versus the KMC 40 coil spring.  The shorter spring reduces slamming of the bolt carrier against the barrel extension.  The ArmaLite buffer places a slight preload on the spring, reducing the need for the longer spring.  The ArmaLite spring and buffer are usable in the KMC SR25.

UPPER RECEIVER:  Three versions are built: the AR-l0A4 version topped off with a Picatinny Rail (Weaver style) sight mounting surface, the AR-l0A2 version bearing an integral carrying handle, and a slightly modified AR-10A2 version made for the trigger-style charging handle of the AR-10B. ArmaLite produces a removable front sight and removable carrying handle for use on the Picatinny Rail surfaces. 

ArmaLite upper receivers are forged rather than extruded or cast, and include an integral case deflector.  ArmaLite’s uppers are available for the KMC Lower Receivers.  KMC use of an extrusion instead of a forging requires a flat-sided receiver form; the core of the ArmaLite upper receiver is generally cylindrical, and quite similar to that of the M-16.  No Forward Assist is provided.   It is not considered necessary given the more powerful operating spring used.

BARREL NUT:  Two barrel nuts are used on the ArmaLite rifles: a simple ring style on the target models, and a rather more complicated nut for the -A2 and -A4 models that adapts standard M16 handguards to the AR-10. 

The relatively complex -A2 barrel nut was designed to allow a spring loaded slip ring to be used to hold standard rifle handguards. This system is preferable to the early jam nut style handguard ring used on 1960 era AR-10s and on current SR-25s because the jam nut system loosen rapidly as the barrel expands during firing.   ArmaLite abandoned jam nut type handguard rings early in the AR-15 program.

The AR-10(T) barrel nut is a ring that bears internal threads matching those of the upper receiver, and an external diameter matched to the bushing of the handguard.  Because of the need to raise the gas tube, the gas tube holes of the ArmaLite nut are .010 farther from the boreline than those of the KMC part, but they remain interchangeable.

CARRIER:  The AR-10B carrier is virtually identical to that of the SR-25.  It is externally phosphated instead of chrome plated as with the SR-25, but is chrome plated in the gas expansion chamber like the carrier of the M-16.  The carrier key surface is .010 inch farther from the boreline than that of the KMC carrier, and .030 further to the rear for better gas transfer into the carrier.   Like the KMC carrier, the ArmaLite carrier will not allow a standard width hammer to enter the ArmaLite Carrier Assembly.  Initial testing has shown that the carrier and bolt group is interchangeable with the KMC components.

FIRING PIN:  Similar to the KMC firing pin, but with several key improvements.  The firing pin tip bears a short cylindrical surface just behind the tip.  This allows the bolt to have a cylindrical section just behind the bolt face, reducing the possibility of sheared primer material jamming into the opening.  The KMC firing Pin and Bolt have tapered surfaces at this point.

The ArmaLite firing pin has a single flange, and a cylindrical section before it that mounts a firing pin spring.  The firing pin spring greatly reduces the light firing pin strike produced in M-16 type rifles when the bolt locks, and thus reduces the possibility of slam fire.  In addition, it positively prevents incorrect assembly of the bolt carrier assembly.  The KMC firing pin has two flanges which makes it possible to install the firing pin retainer in front of the second flange: the carrier assembly appears correctly assembled but the firing pin cannot strike the primer.

BOLT:  The Bolt is similar to that of the KMC model.  Like the KMC bolt, it bears tapered lugs with increased root width for added strength. Six locking and one Safety Lug (patent pending) are used.  This system provides greater strength than the seven conventional locking lugs of M16 type bolts.   The firing pin hole has been reduced in diameter to prevent primer material from extruding into the firing pin hole if cartridge pressures are high or if primer material is soft.

GAS RING:  A single three-turn McFarland gas ring is employed rather than the three piston rings of the early AR-10s and the SR-25.   This construction cannot suffer reduced gas energy due to alignment of the splits of the conventional rings.  The McFarland ring functions perfectly on the SR-25 bolt.

EXTRACTOR:  The extractor is patterned after that of the prototype ArmaLite AR-16 rifle.  It doesn’t interchange with either early AR-10 or SR-25 extractors.

EJECTION PORT COVER:  Generally identical to that of the M-16, but a half-inch longer to accommodate the 7.62mm cartridge.

EJECTION PORT COVER SPRING:  Due to the close fit of the ejection port cover to the upper receiver, insufficient space was available to accept a standard M-16 ejection port cover spring.  In addition, the wider ejection port would result in the short leg of the standard spring protruding into the ejection port, and could injure the fingers of a firer hand loading or clearing a cartridge or case.  A special spring was designed which fit the existing reinforcement ribs of the ejection port cover to solve the clearance problem, and with a shorter leg to prevent injury to the shooter.

GAS BLOCK:  Steel, bearing a short segment of Picatinny Rail mounting surface for sights or other accessories.  It may be loosened to zero the removable sights.  It is one, cross-slot longer than the KMC gas block to allow more secure attachment of hardware.

GAS TUBE:  .240 Longer than the M-16 gas tube used by KMC to allow use of standard handguards and the spring loaded slip ring of the AR-10A2 and -A4 (see above).

BARREL:  The AR-10(T) barrel is produced from a match grade, stainless steel barrel blank.  The standard AR-lOA2 and -A4 barrels are button rifled and chrome lined, with stainless barrels available as an option.   Target barrels are 24 inches long; standard barrels are 20 inches long.   Carbine length barrels are 16 inches long.  Replacement of the barrel requires special tools, and should be performed only by ArmaLite personnel or a qualified gunsmith.

RECOIL CHECK:  The AR-lOA2, A4, B, and all carbines, bear a three slot recoil check on the muzzle.  The recoil check is threaded and pinned into place, and the end of the pin is welded to the check to prevent its removal.

HANDGUARD:  The AR-10(T) employs a tubular fiberglass handguard assembly.  The A2 and A4 versions accept either M16Al or M16A2 style handguards. 

The AR-10(T) handguard is constructed differently from the KMC version.  The handguard-locating stud is pressed into a recess in the handguard bushing rather than riveted to the tube through the bushing. Instead of a bare fiberglass end, the ArmaLite handguard terminates in an aluminum sleeve that protects the fiberglass, reinforces it, and provides a mounting point for the sling swivel.

LOWER RECEIVER:  The AR-10 lower receiver follows the familiar lines of the M16 much more than that of the SR-25.  The longer ArmaLite magazine catch button need not be mounted in a depression in the side of the receiver as is the M16 button used in the SR-25.  The AR-10B button can be operated without removing the hand from the grip.  The familiar fence protecting the M-16 magazine catch from accidental actuation has been built into the AR-10B. 

The AR-10B series Lower Receiver is not interchangeable with either the SR-25 receiver or those of the first generation AR-10 or AR-10A. ArmaLite Lower Receivers are NEVER sold separately.  All ArmaLite rifles are factory built.

TRIGGER MECHANISM: The AR-10(T) employs the ArmaLite two-stage match trigger as standard.  The B, 'A2, and 'A4 models employ the single stage trigger assemblies, but may be fitted with the two stage trigger mechanism as an option.

BOLT CATCH:  The SR-25 employs the M-16 bolt catch.  This device was designed to stop the small M-16 bolt carrier assembly, not heavier AR-10 bolt carrier assembly.  The AR-10 bolt catch is larger and stronger than that of the M-16/SR-25 to function with the heavier AR-10 components. It is designed to automatically catch the bolt when pressed upward by the bolt catch trip in the follower.

MAGAZINE CATCH BUTTON:  Identical to that of the M-16, except that it is somewhat longer to allow it to protrude from the side of the wide AR-10 lower receiver.

MAGAZINE CATCH:  An improved, machined catch with a longer engaging surface to capture the magazine better, elimination of a bevel to prevent excess upward movement of the magazine, and a longer shaft to span the wider receiver.

MAGAZINE:  The AR-10 magazine is based on the magazine of the M14 Rifle.  The M-14 magazine is well proven and available in large numbers.  It is both stronger and more reliable than that of early AR-10 and similar magazines.  It is made in 10 round capacity. 

The AR-10B magazine does not bear a latch plate on the rear surface like that of the M-14 magazine.  The lower receiver, however, has a clearance cut up the back of the magazine well to allow insertion of an M-14 magazine.  A 20 round M-14 magazine can therefore be modified to function perfectly in the AR-10.  This modification requires changes to the feed lips cutting a magazine catch opening cutting a slot down the back of the magazine replacement of the follower with a new follower addition of a bolt catch trip and spring a slight change in the follower spring.

The most unique aspect of the ArmaLite magazine is the follower.   A top surface similar to that of the M16 magazine has been combined with a form suited to the M-14 magazine box.  A patented spring-loaded plunger has been installed in the follower.  When the follower rises to the top of the magazine box, this plunger pops into the track at the rear of the magazine well and trips the bolt catch, thus holding the bolt carrier assembly to the rear.

The magazine will continue to function in the M-14 rifle, but will not trip the M-14 bolt catch unless the AR-10 follower is replaced with the M-14 follower.   The latch plate at the rear of the magazine (which engages the M-14 magazine catch) may be sufficiently weakened by removal of its upper half that it breaks off, rendering the magazine usable only in the AR-10.

USING M-14 RIFLE MAGAZINES:  The AR-10B magazine is based on the proven M-14/MlA rifle magazine.  A good quality metal 20 round M-14/MlA magazine can be altered to work perfectly in the AR-10B. U.S. GI and imported Chinese magazines are acceptable.  Commercial metal magazines with blue finish or pressed latch plates require replacement of the shell.  Fiberglass magazines (i.e. Thermold, etc.) are totally unsuitable for conversion.

ArmaLite sells 20 round magazines when available.  ArmaLite will provide converted magazines for a fee or on a 2-for-1-exchange basis. Magazine conversion kits allow a customer to convert all metal M14/M1A magazines and metric FAL magazines to AR-10B series magazines.

MAGAZINE INTERCHANGEABILITY:  10 round AR-10B magazines and converted 20 round M-14 magazines will not work in early AR-10/AR-10A rifles, or in the current SR-25 rifle.  Magazines for those rifles will not work in the new AR-10B series rifles.  Unless converted with the reversible magazine conversion kit, M-14/M1A magazines should not be used in the M-14 or M1A after modification.