Heckler&Koch MR556A1 - Manual (Page 5)

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NOTE: Both the pivot pin and the rear takedown pin are special precision “target rifle”
pins. For disassembly and reassembly, a center detent in the shaft of the pin must be
engaged under pressure and held to move the pins back and forth. DO NOT FORCE
THE PINS. Forcing the pins could damage the rifle and such damage is not covered
under the HK warranty. Contact HK Customer Service with any questions, TEL: 706-
568-1906, EMAIL: [email protected].
17. Magazine – Holds cartridges ready for feeding and provides a guide for positioning
cartridges for stripping and chambering. The magazine provides reload capabilities for
sustained firing. The HK 30-round steel magazine and optional 20-round steel magazine
have a steel follower that will not cant during the feeding process. The spring is stronger
and more reliable than standard magazine springs. The MR556A1 can be used with other
high quality magazines of all capacities.
18. Magazine Catch/Magazine Release – Holds the magazine (in place) in the magazine
well and allows the operator to release the magazine and remove it from the firearm. The
magazine catch is activated automatically when the magazine is properly inserted into the
magazine well. The magazine catch is spring loaded and when the button is depressed,
the magazine will drop free from the firearm when this control is activated.
19. Lower Receiver – Contains the fire control system for the firearm. The lower receiver
contains the hammer, trigger, disconnector, sear, magazine well, magazine catch/release,
bolt catch/release, selector/safety lever, buttstock assembly with buffer and recoil spring,
pistol grip and trigger guard. The lower receiver is made from an aluminum forging and
provides a beveled magazine well to ease reloading.
20. Pistol Grip – The pistol grip is made so it can be used by a left or right handed shooter.
It has a trapdoor on the bottom so extra batteries, small tools, or other accessories may be
stored. Other AR-type pistol grips can also be installed.
21. Rear Takedown Pin – When the rear takedown pin is depressed, it allows the upper
receiver to pivot on the front pivot pin. This pin is captive and should not be removed. Like
the pivot pin (front takedown pin), the rear takedown pin has a locking shaft that needs to
be pressed while the pins are depressed and engaged for disassembly and reassembly.
22. Buttstock Release Lever – The buttstock release lever locks the telescopic buttstock to
the tubular rear buttstock extension. The buttstock is adjustable in length in 6 positions.
23. Rear (Diopter) Sight – proprietary HK sight used on many HK firearms, attaches to the
MIL-STD-1913 top rail of the upper receiver.
24. Front Sight – proprietary HK sight used on many HK firearms, attaches to the MIL-STD-
1913 top rail of the Free Floating Rail System (FFRS).
25. Front (Flip up) Sight – proprietary HK sight attaches to the gas block, folds down when
not in use.
INTERNAL PARTS NOMENCLATURE
Muzzle – The muzzle is the area at the end of the barrel and the last area that the projectile
touches before it exits the bore.
Bore – The part of the barrel that the projectile travels through from the chamber to the
muzzle and contains lands and grooves which make the projectile rotate.
Chamber – The chamber is the area where the cartridge is seated and the initial phase of
firing occurs.
7. Trigger – When activated by the operator, initiates the firing sequence. Located inside
the lower receiver, the trigger when squeezed with the firearm loaded and off “safe” will
fire the gun, “SEMI-AUTOMATIC” will fire one round each time the trigger is pulled. The
MR556A1 uses a proprietary HK two-stage trigger for optimal accuracy.
8. Trigger Guard – The trigger guard protects the trigger. It can be lowered by depressing
the detent with a pointed implement, allowing the use of heavy gloves or mittens to be
used to fire the rifle.
9. Bolt Catch/Bolt Release – The bolt catch/release is manually operated. When the bolt
carrier is pulled fully to the rear, the bottom of the bolt catch/release is pressed by the
operator and then the bolt will be captured. To release the bolt, simply press the top of
the bolt catch/release. The bolt catch/release is also automatically activated when firing
the last round in a magazine.
10. Sliding Buttstock – Attached to the firearm via the receiver extension. The sliding
buttstock is ambidextrous and adjustable to 6 positions for length of pull by pressing in
on the rear portion of the Release Lever. It also contains the rear sling attachment points.
Many optional buttstocks can be attached to the MR556A1 as long as they fit on the
Receiver Extension (Buffer Tube), which has MIL-SPEC cross sectional dimensions.
11. Forward Assist – Ensures that the bolt is fully closed and locked. The forward assist is
used when a round is not fully chambered. You can push on the forward assist until the
bolt seats the cartridge fully into the chamber. It will return to its rearward position by
spring pressure. It can also be used to silently load the firearm.
12. Case Deflector – Built into the upper receiver, deflects the cartridge cases away from
the operator.
13. Ejection Port Cover Assembly – The ejection port cover is attached to the upper
receiver and is spring loaded. It will open automatically if the charging handle is pulled
to the rear or if the firearm is fired and can be manually closed at any time. It closes over
the ejection port to prevent sand, dust, etc. from entering the chamber. It should remain
closed when the firearm is not being fired.
14. Barrel Assembly – The chamber accepts the cartridge for firing and directs the
projectiles down range. The barrel is cold hammer forged and has standard land and
groove rifling with a 178 mm (1 in 7 inch) twist. The barrel is pressed into the upper
receiver assembly and held in place by the threaded barrel nut.
15. Locking Screw, Free Floating Rail System (FFRS) – The Free Floating Rail System is
fastened with the locking screw to the forward extending barrel locking bushing. Fastening
and positioning of the Free Floating Rail System on the bushing is positive and ensures
maintenance of zeroing on any sight system which has previously been attached to the rail
after removal and remounting of the rail. The Locking Screw is captive and should not be
completely removed, it can be tightened and loosened by using a 5 mm Allen wrench.
16. Pivot Pin – The pivot pin allows the upper receiver to pivot when the rear takedown pin
is depressed. When both the pivot pin and rear takedown pin are depressed, the upper
and lower receiver groups can be separated. The pivot pin is captive and should not be
removed. Both the pivot pin (front takedown) and rear takedown pin have locking shafts
that need to be pressed while the pins are depressed and engaged.