Problem Solver: HK Front Sight Tower 1913 Rail Mount

By Alton P. Chiu

The Heckler & Koch (HK) family of roller-delayed firearms predates practical weapon lights and modular accessory attachments. The large and unwieldy lights of yesteryear were cumbersome and relegated to specialist roles. The advent of LED and high-energy-density lithium batteries enabled AR-15 users to modularly and ergonomically employ weapon lights. MP5 users must resort to dedicated forends, such as the SureFire 328LMF-A, or upgrade to modern handguards featuring 1913, M-LOK, KeyMod and even Spuhr Interface attachment systems. These solutions are either non-modular or costly.

HKParts solves this predicament with a $40 lightweight aluminum 1913 rail mount attaching to the front sight tower (http://www.hkparts.net/shop/pc/Picatinney-Rail-Mount-For-Front-Sight-Tower-91p16672.htm). It ergonomically positions pistol weapon lights, such as the SureFire X300U and Streamlight TLR-1, on an MP5K with ambidextrous activation. However, it requires fitting to function with a PTR 91 and three-lug muzzle devices such as a suppressor.


To install, one positions the mount on the ejection port side of the front sight tower. Its two legs bracket the tower to align the threaded hole on the mount with the handguard hanger. The supplied lock washer and screw secured the mount snugly against the handguard using a 4mm hex key. The installation was quick and effortless.

The mount was designed for HK dimensions and exhibited excellent fit on the MKE Z-5P (an MP5K license-produced in Turkey). However, the front leg of the mount required some fitting for a PTR 91 (a G3 clone produced in South Carolina). The legs served to align the mount with the bore axis and to prevent rotation during installation. As an additional benefit, the author found that the clamping force eliminated play in the U.S.-made handguard. However, the mount prevented a free-floating barrel.

Fitting for Three-Lug

The front leg, as designed, extends too far forward and prevents the installation of three-lug muzzle devices, as one cannot depress the device far enough to rotate into the locked position. This author was unable to mount his Obsidian45 suppressor from Rugged Suppressors, and he attempted to address this by removing front leg material. However, this modification left only a thin triangular stub of questionable structural integrity. As such, the front leg was removed altogether. The corner of the mount was also rounded such that the suppressor could be properly mounted. However, this modification allowed the light mount to rotate as the screw was tightened.

The rear leg of the light mount proved to have slight play against the front sight tower. This allowed the mount to rotate and led to a misaligned weapon light. This author added Vaseline to the tower as a release agent and applied J-B Weld epoxy to the rear leg of the light mount. This additional material removed play and aligned the bore. As an added measure, this author added a neoprene washer between the handguard and mount for extra friction. Secured in this way, the mount did not rotate during installation and light activation. As designed, the system worked well, the modifications were made to facilitate the author’s ideas.


The mount proved ergonomic for both left- and right-handed users when using handgun weapon lights with two switches, such as the SureFire X300U. A left-handed user can push the top switch forward with his thumb for momentary activation. A right-handed user can push the bottom switch for the same effect. “Constant on” mode was also easily activated by pushing either button toward or away from the firearm, although the author preferred the former method. The Streamlight TLR-1 can also be used ambidextrously. For momentary activation, a left-handed user can push the top switch toward the firearm, while a right-handed user can push the bottom switch away. The author felt that the former action was more natural than the latter.

Lights with tailcap switches, such as the SureFire Scout, are harder to activate ambidextrously. A light placed in-line with the mount positioned the tailcap too low for the left-handed user and too high for the right-handed user. An offset mount, such as the SureFire RM45, improves ergonomics for one shoulder. However, without reconfiguration, the light becomes nearly inoperable ambidextrously.

A mounted suppressor can cast a shadow to the left of the beam pattern. The Obsidian45 suppressor with a three-lug attachment adds 180mm to the overall length of the MKE Z-5P. Its shadow just about touches the spotlight portion of the X300U’s beam pattern. However, the blockage is minor, whether the user is “splashing” the light or directly illuminating the target. When used indoors, the 600-lumen output is more than sufficient to provide illumination. The reflected light sufficiently fills in the suppressor shadow so that target detection is still possible. When used outdoors, one must use the spotlight portion of the pattern for positive target identification at any rate. That portion is left unobstructed by the lengthy suppressor. As such, the shadow effect is negligible.


The 1913 rail mount available from HKParts is a worthwhile addition to the family of roller firearms. It provides a weapon-light interface while preserving the classic aesthetics. On the MP5K, it ergonomically positions handgun lights such as the SureFire X300U and Streamlight TLR-1 for both left- and right-handed use. The low cost is offset by the need for fitting to accommodate the PTR 91 and three-lug muzzle devices.

This article first appeared in Small Arms Review V22N4 (April 2018)
and was posted online on February 23, 2018


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