Heckler&Koch MR556A1 - Manual (Page 27)

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SECTION 12 — MR556A1 UPPER RECEIvER KIT SUPPLEMENT
This kit has been designed and manufactured to Heckler & Koch’s exacting standards of
excellence and is intended for use in, and with, MIL standard buffer tubes, as are provided
with the HK MR556A1 rifle. Due to the precise nature of the components of this kit, they
must be installed as a complete kit to ensure proper operation. The MR556A1 Upper
Receiver Kit (URK) consists of an MR556A1 upper receiver/barrel assembly, complete
bolt mechanism, and buffer and action (recoil) spring. The MR556A1 Upper Receiver Kit
is designed to be fully interchangeable with AR-style firearms. MR556A1 upper receiver
kits can be used to retrofit many non-HK AR systems by replacing the aforementioned
components with MR556A1 URK components.
NOTE: Due to the shortened length of the buffer and action spring, these parts do
not work in fixed stock AR-type rifles. Additionally, the MR556A1 URK 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.
Fig. A – MR556A1 Upper Receiver Kit Components and non-HK Lower Receiver
ASSEMBLY
With the upper receiver and barrel assembly removed from the non-HK AR-type firearm,
press in the buffer slightly, depress the retainer (Fig. B), and release the buffer, removing
the non-HK buffer and action spring (Fig. C).
Fig. B – Depress Buffer Retainer Fig. C – Removing non-HK Buffer
and Action (Recoil) Spring
Ensure that the hammer is in the down (cocked) position before attempting to install the
HK buffer and action spring. Mount the HK action spring onto the buffer and insert the HK
buffer and HK spring into the receiver extension. Push back on the buffer far enough until
the buffer is retained by the spring loaded buffer detent (Fig. D).
Fig. D – Installing HK Buffer and Action (Recoil) Spring
CAUTION: This kit is designed operate with a MIL standard M4 buffer tube. Failure to
do so, or to install the complete kit as provided, will void your warranty. Always have
a competent gunsmith assemble your rifle kit and headspace your barrel before first
use. Use of an incorrectly assembled rifle, or using incorrect or modified parts, can
result in damage to the rifle, serious injury or death.
The HK bolt mechanism should already be installed in the HK barrel assembly/upper
receiver, if not, push the charging handle and bolt group completely into the upper
receiver until the locking lugs on the bolt head engage the locking on the barrel extension
and the latch on the charging handle engages the oval locking surfaces on either the left
or right hand sides of the upper receiver (see pages 32-34 in the MR556A1 Operator’s
Manual for additional information).
Join the MR55A1 upper receiver/barrel assembly to the lower receiver (Fig. E) of the high-
quality AR15/M4/M16 type firearm by:
1) Align the upper and lower receiver
Fig. E – Installing the MR556A1 Upper Receiver on a non-HK Lower Receiver