Heckler&Koch MR556A1 User Manual

Page 6

ASSEMBLY GROUPS

Fig. 2 – MR556A1 Major Assembly Groups

1. Lower Receiver Assembly - contains the Buttstock assembly (holds buffer and recoil

spring), pistol grip, selector lever, trigger group, trigger guard, magazine catch/

release, bolt catch/release, magazine well, pivot pin and take down pin.

2. Buffer and Recoil Spring

3. Charging Handle Assembly

4. Upper Receiver Assembly - contains the forward assist, case deflector, ejection port

cover assembly, Free Floating Rail System, gas block (with folding front sight point),

compensator, barrel, gas piston and piston rod.

5. Bolt Carrier Assembly

6. Magazine Assembly

Bolt and Carrier Assembly – Provides stripping, chambering, locking, firing, extraction, and

ejection of the cartridges using the recoil spring and projectile propellant gases for power.

These parts are made of the highest quality steel.

Bolt – The bolt locking lugs rotate and lock the barrel and bolt together prior to cartridge

ignition.

Extractor – The extractor grabs the rim of the cartridge case and pulls the case out of the

chamber. The extractor spring and buffer provide constant pressure on the rim until the

case is ejected.

Ejector – The ejector pushes against the cartridge base under constantly spring tension

and once the cartridge or case is extracted and clears the front of the ejection port, the

cartridge or case is ejected from the firearm.

Cam Pin – The cam pin allows the bolt to cam inside the bolt carrier, which allows the bolt

to lock into the barrel extension when fully forward and cams to unlock when the carrier is

pulled to the rear.

Firing Pin – The firing pin will strike the primer and initiate the firing sequence once the

trigger is pulled. The firing pin will only protrude through the face of the bolt when the

bolt is fully locked forward.

Firing Pin Safety Spring – Prevents an accidental discharge in the event of dropping or

jolting the gun.

Firing Pin Retaining Pin – Retains the firing pin in the bolt carrier.

Recoil spring and buffer – The recoil spring provides constant pressure on the bolt carrier

and holds the bolt in a locked forward position. The buffer has tungsten granules that act

as a anti-bounce back mechanism. One type of buffer serves all barrel lengths.

Buffer detent – Captures the buffer and spring and when depressed, releases them for

removal.

Hammer – The hammer, when released by the trigger, strikes the firing pin. The hammer

returns when the bolt carrier pushes back under recoil, the hammer makes contact with

the disconnector and will release from the disconnector when the trigger is released. The

hammer remains cocked under spring pressure.

Trigger – The trigger, when squeezed, releases the hammer and returns under spring

tension.

Disconnector – Captures the hammer when it is cocked to the rear and releases the

hammer to the sear when the trigger is released. The disconnector then returns to its

original position under spring tension.