Heckler&Koch MR556A1 User Manual

Page 27

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