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Tech Note |
September 18, 2000
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 designers 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 "classic" model of the new ArmaLite line. It is
patterned after early 1960 era rifles, and is produced largely for collectors. The
charging handle looks like an upside down trigger and is located at the front of the
carrying handle. It comes with special brown furniture duplicating the original color, and
a rear sight elevation window at the rear of the carry handle.
AR-10(T); A premium target model featuring a 24 inch long, medium weight
target barrel, tubular handguard, match trigger, and Picatinny Rail receiver and gas
block.
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.
Each rifle is matched by a similar carbine that is 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 thick, nylon trap door provides 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 flattop AR-l0(T) and 'A4
version topped off with a Picatinny Rail (Weaver style) sight mounting surface, and the
AR-l0A2 version bearing an integral carrying handle. A specially modified AR-10A2 version
is produced for the trigger-style charging handle of the AR-10B.
ArmaLite produces scope mounts and removable sights 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 to accommodate the A2/A4 , 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 like 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. The AR-10
carrier has an opening that allows the shooter to reach in and push the firing pin forward
to ease assembly or disassembly of the carrier group. Initial testing has shown that the
carrier and bolt group is interchangeable with the KMC components.
FIRING PIN: The AR-10 firing pin is 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 in front of 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
patented safety lug 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 reduce the tendency for primer material to extrude 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 generally interchanges with SR-25 extractors, but not 1960 era
AR-10s.
EXTRACTOR SPRING REINFORCEMENT: ArmaLite's exclusive Extractor Spring
Reinforcement Ring is an elastomer reig that surrounds and reinforces the easily stressed
extractor spring. The AR-10, SR-25 and the M-16 all use an elastomer plug inside the
extractor spring.
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.
AR-10T and AR-10A4 GAS BLOCK: Steel, bearing a short segment of Picatinny
Rail mounting surface for sights or other accessories. It is clamped to the barrel and
secured by two clamping screws, which 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.
FRONT SIGHT BASE: ArmaLite uses an exclusive front sight base forging.
Like the Gas Block of the A4 and T models is clamped to the barrel and secured by two
clamping screws, which may be loosened to zero the front sights. It corrects any tendency
for the rear sight to be off-center when the rifle is zeroed.
GAS TUBE: .The AR-10 gas tube is 240 inch longer than the M-16 gas tube
used on the M-16 or the SR-25 to allow use of standard handguards and the spring loaded
slip ring of the AR-10A2 and -A4.
BARREL: The AR-10(T) barrel is produced from a match grade, triple-lapped
stainless steel barrel blank. The standard AR-10A2 and -A4 barrels are button rifled,
double-lapped, and chrome lined, with triple-lapped target grade 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-10A2, A4, B, and all carbines, bear a slotted 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. The SR-25 does not generally bear a
recoil check or muzzle brake. The recoil check provides excellent recoil control.
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.
The standard grade SR-25s use modified GI handguards that are held in position by a
threaded jam nut, or collar much like the 1960 vintage AR-10s. The ArmaLite A2 and A4
models use M-16 style spring loaded slip rings. The slip ring system allows the barrel of
the rifle to expand and contract during firing without stressing the barrel.
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. Eagle Arms brand AR-10 lower receivers
are produced for commercial sale to customers who want to build their own rifles.
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. The SR-25
uses either a special two stage trigger or a standard trigger that engages a stop screw to
reduce the depth of the first stage.
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.
BUTTPLATE: The AR-10 is equipped with a round edged M16A1 type buttplate
for shooting comfort, not the sharper edged buttplate used on the M16A2 and ArmaLite M-15
rifles. ArmaLite's checkered trapdoor protrudes slightly from the plate to help the plate
grip the shooter's shoulder better than the M16A2 buttplate does.
MAGAZINE CATCH BUTTON: The AR-10 uses a magazine catch button that is
similar to that of the M-16, except that it is somewhat longer to allow it to protrude
from the side of the wider AR-10 lower receiver. The SR-25 uses the same button as the
M-16.
MAGAZINE CATCH: The AR-10 uses 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. The SR-25
uses a conventional M-15 magazine catch successfully.
MAGAZINE: The SR-25 uses a magazine copied from the early AR-10
"Waffle" magazine. That early magazine was intended to be disposable, and it
didn't stand up to repeated or heavy use. The new AR-10B 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. Newly made 20 round magazines are available to law enforcement
officers and organizations. 20 round magazines for commercial customers are produced by
converting "pre-ban" M-14/M1A magazines to AR-10B 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.
RATE OF FIRE: The same as that of the M-14 rifle in rounds per minute for
a maximum period of:
2
minutes………………………………………..40
rounds per minute
10
minutes………………………………………30
rounds per minute
20
minutes………………………………………20
rounds per minute
30 minutes or
more……………………………..13 rounds per
minute
BARREL LIFE depends on how fast the rifle is fired, and how accurate the
shooter insists it must be. Shooting the rifle as fast as the trigger can be pulled can
ruin the barrel in just a few hundred rounds. Firing at controlled rates (see above) will
allow a standard-grade AR-10 barrel to last for 10,000 to 15,000 rounds with accuracy
acceptable for Police use. Target shooters fire at very moderate rates, but their higher
accuracy requirements dictate 7.62mm barrel replacement by about 7,000 match rounds.
MAW
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