The MR556A1 Upper Receiver Kit includes a complete upper receiver/
barrel assembly, bolt carrier assembly, buffer, and buffer spring allowing
Mil-Spec 5.56 mm AR-style rifles to be retrofitted to HK’s pusher rod system.
The Upper Receiver Kit is the core of the HK 5.56 mm proprietary gas piston
operating system. It employs a piston and a solid operating “pusher” rod in
place of the gas tube normally found in AR15/M16/M4-style firearms.
The same operating system is used in the combat-proven 5.56 mm HK416
as well as the G36, HK417, and a variety of other Heckler & Koch arms. The
MR556A1 Upper Receiver Kit is designed to be fully interchangeable with all
high quality AR-style firearms, simply by attaching it to an AR lower receiver
using the standard takedown pins.
Due to the shortened length of the HK buffer and HK (action) buffer spring,
these parts do not work in fixed stock AR-type rifles. Installation of MIL-SPEC
hammers in non-HK lower receivers are required for reliable function.
Additionally, the MR556A1 Upper Receiver Kit does not work in lower
receivers with fully automatic or selective fire mechanisms.
However, the HK416 Upper Receiver Kit (available to military and law
enforcement customers) is designed to function with fully automatic and
selective fire lower receivers.
The HK piston driven operating system 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.
Designed to function with a wide variety of high quality 5.56 x 45 mm
ammunition, the MR556A1 Upper Receiver Kit 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 Upper Receiver Kit is produced in the USA using American
and German made components. The Upper Receiver Kit uses a German-made
barrel manufactured by Heckler & Koch’s famous cold hammer forging
technology. The highest quality steel is used in this unique manufacturing
process creating a barrel that provides superior accuracy and long service life.
The unchromed 16.5-inch 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. A thick,
heavy contour also contributes to its excellent accuracy.
The Upper Receiver Kit also includes the HK Free Floating Rail System
(FFRS) handguard with its four MIL-STD-1913 “Picatinny” rails. This premium
machined alloy handguard allows all current accessories, sights, lights, and
aimers used on M4/M16-type arms to be fitted. The HK rail system can be
installed and removed with simple tools and returns to zero when reinstalled.
The Upper Receiver Kit also has a Picantinny rail machined into the top of
its upper receiver for mounting optics and mechanical sights.
HK MR556 Upper kit includes:
- Complete MR556 Upper Receiver/Barrel
Assembly HK 16.5 inch cold-hammer forged barrel, upper receiver with
forward assist, extended convertible charging handle, HK Free
Floating Rail System handguard
- Complete MR556 Bolt Group Assembly
- HK Proprietary Buffer
- HK Proprietary Buffer (Action)
Spring
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