Tinkering with an FN PS90 Bullpup Rifle

posted on June 29, 2018

The FN PS90 is a unique firearm design in its own right, and when I acquired mine I had no intention of changing things. Nevertheless, sometimes projects evolve in an organic way. The following is one such example and is representative of the seemingly myriad solutions for updating the PS90.

Ring Sights MC-10-80 Black Reticule Secret Service Model

Perhaps like many, my entry point toward modifying my PS90 started at the sights. Mine came with a removable Picatinny rail unit that attached to the upper receiver. Instead of fitting a sight to the rail, I opted to remove it in favor of a Ring Sight with black reticle (1., $300) from Strike Industries. I wanted a small, rugged sight that sat as close to the bore axis of the firearm as possible. In fact, for many PS90s, the Ring Sight unit has been offered as standard equipment.

M1 BullPup: REM-T3B

The black Ring Sight has a shortcoming though. That is, when the sight overlays a dark target in daylight, it can be hard to make out the reticle. Fortunately, there is a solution for this problem. M1 BullPup makes an 11-step-adjustable LED unit called the REM-T3B (2., $40) that can be fitted to the base of the sight to illuminate the T-Bar in the reticle. Once fitted, the sight can be re-installed on the PS90 and the illumination dial can then be mounted through the sling point in the upper receiver. M1 BullPup makes a range of such units to fit the other Ring Sight units, including the White Ring and Green Dot models.

K&M Aerospace: The Handler, K&M Lightning

Talking with Chris Roenker, of M1BullPup, led me to K&M Aerospace to learn more about its extended charging handle, The Handler (3., $50). The aluminum replacement can be installed on either side of the PS90. It greatly enhances the operator’s purchase when folded out, as it provides 13/4" of surface to pull back on. Once the PS90 is cocked and the charging handle released, it will return to a forward folded position. In the folded state, The Handler protrudes only 1/4" more than the original.

K&M Aerospace also makes an improved spring called the K&M Lightning (4., $30) that installs in the PS90 hammer pack. While the stock trigger pull-weight of the PS90 is 6 lbs., 13 ozs., the K&M Lightning brings that pull weight down to between 4 and 4 lbs., 8 ozs. Users can install the spring themselves or, for an extra charge, send their hammer packs to K&M Aerospace where it will install the new spring.

At this point I realized I was overlooking the obvious—the sling. Senior Executive Editor Brian Sheetz steered me toward The Wilderness and its Giles Sling (5., $56). The Giles incorporates a quick-release buckle with two sections of high-quality webbing that attach through the slot in the lower rear of the buttstock and through a sling point defined by the owner toward the front of the PS90’s receiver.

The upper receiver of my PS90 has threaded holes tapped in the sides forward of where the magazine rocks in. The holes allow the user to mount a short piece of Picatinny rail. I turned to Damage Industries for its 1913 Side Rail (7., $35), a six-slot unit with a QD socket. Equipped with this new rail, I had expanded my options. I also opted for Damage Industries Quick Detach Swivel with 1.25" loop (6., $13). The 1.25" loop accommodates the forward webbing of the Giles Sling.

(top r.) Damage Industries: Quick Detach Swivel with 1 1⁄4" Loop (top r.) Damage Industries: 1913 Side Rail with QD Socket (btm.) Parker Mountain Machine QD Sling Mount

As a matter of course, I found that I needed to reverse the suggested mounting of the rail to accommodate the illumination dial for my Ring Sight, which places the QD socket toward the front of the receiver.

During my search I found one other interesting solution in Parker Mountain Machine’s PS90 QD Sling Mount (8., $27). The CNC-machined, anodized aluminum mount is made to install in the user’s choice of counterbored hole already in the PS90’s lower frame. The mount is eccentric, allowing the user to rotate the socket so that it’s base lies flush to the bullpup’s surface. The QD socket provides another sling attachment option for those looking to keep their sling attachment off of the PS90’s upper.

Finally, as I like to save my spent cases, I picked up an FN-PS90 Fired Case Collector (9., $57) from Brownells. The PS90 lower is designed to accept this bag that catches the downward-ejected cases from the receiver and can hold up to 50 cases.

P.S. The versatile and trim platform of the PS90 allowed me to make all of these upgrades without permanent modification to the firearm. I would like to reinforce the point, however, that there are many directions one might take to achieve results that will personalize the PS90 experience.



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