Heckler&Koch MR556A1 User Manual
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SECTION 1 — INTRODUCTION
A direct descendent of the HK416, the MR556A1 is a semi-automatic rifle developed by
Heckler & Koch as a premium level commercial/civilian firearm with match rifle capability.
Like the HK416, the MR556A1 is a major product improvement over conventional AR-type
carbines and rifles. The MR556A1 uses the HK proprietary gas piston operating system,
employing a piston and a solid operating “pusher” rod in place of the gas tube normally
found in AR15/M16/M4-style firearms.
This method of operation virtually eliminates malfunctions that are common to direct
impingement gas systems since hot carbon fouling and waste gases do not enter the
receiver area. This is the same key feature critical to the success of the HK416 rifles used
by leading military and law enforcement customers. The MR556A1 is manufactured in the
USA using American and German made components. The MR556A1 uses many of the
same assemblies and accessories originally developed for the HK416 including the HK
free-floating four-quadrant rail system.
The HK Free Floating Rail System (FFRS) handguard has four MIL-STD-1913 Picatinny
rails and allows all current accessories, sights, lights, and aimers used on M4/M16-type
arms to be fitted to the MR Series. The HK rail system can be installed and removed with
simple tools and returns to zero when reinstalled. The MR556A1 also has a Picantinny rail
machined into the top of its upper receiver for mounting optics and mechanical sights.
Unlike earlier prototypes, major subassemblies, including the MR556A1 upper receiver/
barrel assembly, are fully interchangeable with other high quality AR-style firearms.
MR556A1 upper receiver kits can be used to retrofit competing legacy AR systems.
Like the famous HK416, the MR556A1 uses a German-made barrel produced by Heckler
& Koch’s famous cold hammer forging process. The highest quality steel is used in this
unique manufacturing process producing a barrel that provides superior accuracy and
long service life. But unlike the HK416, the MR556A1 does not use a chrome-lined barrel.
Chrome-lining can sometimes mask bore imperfections and negatively affect accuracy. For
the new series of Heckler & Koch semi-automatic MR rifles, HK designers and engineers
believe best accuracy comes with an unlined bore.
Used with the HK Free Floating Rail System, the barrel does not touch the handguard,
ensuring the barrel is truly free-floating for maximum accuracy. The unchromed barrel
has an internal profile of 6 lands and grooves with a 1 in 7 inch (178 mm) twist. It is also
moderately “swagged” with a slightly smaller internal diameter at the muzzle end than the
chamber end. This feature has a positive effect on bullet accuracy and velocity. The thick,
heavy contour 16.5-inch MR556A1 barrel also contributes to its excellent accuracy.
A precise enhanced two-stage trigger adds significantly to the accuracy of the MR5556A1
and an extended latch charging handle makes operating the rifle with a telescopic sight
easier. The retractable buttstock can be locked into a solid, non-moving position by
tightening an Allen screw in the bottom of the stock. Special target rifle takedown pins
ensure the upper and lower receivers are firmly mated for accurate performance.
Designed to function with a wide variety of high quality 5.56 x 45 mm ammunition, the
MR556A1 uses a C.I.P. dimensioned chamber (a chamber with dimensions specified by the
Commission Internationale Permanente). The MR556A1 also functions reliably with most
quality Caliber .223 Remington ammunition. The MR556A1 is being produced with the
same commitment to quality as German-made HK firearms. By using joint U.S./German
manufacturing, Heckler & Koch leverages the relative strengths of two continents to
make the preeminent AR-type firearm, combining the design innovations of the USA and
Germany into one new product.
SECTION 2 — NOMENCLATURE & DESCRIPTION
Fig. 1 – MR556A1 Right and Left Views
1. Compensator – Bird cage style flash hider is screwed to the end of the barrel and helps
compensate for muzzle rise during firing.
2. Gas Block – Directs the expended gas from the gas port hole in the barrel and pushes
the piston rearward, pushing the piston rod and bolt carrier rearward (defined in more
detail in the cycle of operation). Contains an attachment point for the optional HK folding
front sight.
3. Free Floating Rail System (FFRS) – The HK FFRS is a free-floating detachable rail system
with four-quadant MIL-STD-1913 (Picatinny) rails that can mount a variety of accessories
including sights, scopes, a forward grip, lights, laser aimers, bipods, and sling hardware.
The bore sight alignment of aiming devices is repeatable (no re-zeroing required) when the
Free Floating Rail System is removed and reinstalled on the firearm. The Free Floating Rail
System is removed and reinstalled with a 5 mm Allen wrench.
4. Upper Receiver – The upper receiver is a flat-top design with an integral MIL-STD-1913
(Picatinny) rail that provides support for the barrel, FFRS, gas system, and bolt carrier
assembly. It has a case deflector and ejection port. It connects to the barrel and barrel
socket. The piston rod is guided through the upper receiver by a bushing and activates the
bolt carrier. The charging handle is a part of the upper receiver as is the forward assist and
ejection port cover. The upper receiver is made from an aluminum forging.
5. Charging Handle – Allows the operator to chamber or clear a round and cock the
firearm. It also provides for initial charging of the firearm. The charging handle is locked in
the forward position during firing to prevent injury to the operator. An enhanced charging
handle with an extended catch is available on the MR556A1.
6. Selector/Safety Lever – Allows the operator to select the mode of fire and place the
firearm on safe. The selector lever is manually activated and unlike competing AR systems,
can be switched to safe even if the hammer is not cocked.