FNX-9/40 - Model in work, and a brief analysis (Patreon)

My recovery at home has been going well. It's nice to be away from work for once. One of the projects I have been working on (and has been on hold for so long) is the FNX-9/40.

It came into my hands when an anonymous someone generously donated a kit to me.

The FNX is a polymer-frame hammer-fire pistol. It's a precursor to the striker-fire FNS, with the former being released in 2009, and the latter in 2011. While it bares some similarities (and even some cross-compatible parts), they are fundamentally different guns.

Something Different

The FNX is a curious example of metal rails in a polymer gun. Compared to a Glock, where the metal rails are injected into the frame itself, the FNX has the rails pinned or held to the frame as removable plates.

Retrospect tells us that FN Herstal moved to incorporating rails into a locking block or trigger housing later on with the FNS and FN 509 series of pistols. The FNS had its locking block be a singular, beefy chunk of metal, with the 509 later optimized to be much lighter in weight and materials. (Note the reduced rail surface area.)

FNS Locking Block

FN 509 and FNS-9C/40C Locking Block

The rails on the FNX are pesky to install. It might look as straightforward as just putting them in place and installing everything else. Not exactly so. The rear rails are prone to falling out; the front rails are an entire assembly you must put together, and pinch while inserting on top of the trigger, slide release, and slide release spring. FNH's change to a singular, monolithic [locking block with] rails is a welcome one I'm sure for their assembly people.

While the monolithic locking block is easier to install and put together in a gun compared to the plates, it does have a drawback that's only apparent for the home armorer or gunsmith. 

The locking block is held to the frame using two pins- the trigger pin, and a dedicated locking-block pin. The latter pin is a simple pin, not a spring one. Since the pin, being a single piece of metal, flattens out at the head it's struck on, the pin will enter the frame, then locking block, and then rest its stricken side into the frame. Normally, this isn't a problem for armorers or assembly people- it's a one time install before the gun ships and isn't designed to be removed , and armorers have a jig to use to remove the pin (for FN 509's at least).

For engineers like TMS and myself, however, this was a startling discovery. I had made the mistake of reaming in the front pin, and eventually had to cut and file it flat, leaving the pin in there forever, and depending on the (more than capable) trigger pin and frame geometry to hold the locking block in.

The rear trigger housing assembly is complex for both the FNX and FNS, but I think the rear rail plates are all but required for the design. The hammer-fired FNX hammer assembly somehow manages to compact a hammer spring (which is typically a hammer strut down the beavertail!), with a de-cocker and safety into one assembly. It accomplishes this with some small cams and spring-loaded arms for the various functions. The design that FN chose for the rear housing had these cams "sit" in the housing, only pivoting when the safety lever is moved, leaving little room for an integrated rail section.

(I must note that if you have a kit, under no circumstances should you let the two halves of the rear assembly seperate. The housing is held together by friction from the center cylinder, binding the mainspring in place with the two sears, while the outer housing is accompanied by two tiny spring-loaded arms.)

Some Similarities, But Not Many

While the FNS and FNX operate radically differently, there are some similarities in design.

Existing FNS owners would be pleased to hear that FNX magazines and FNS magazines are cross-compatible, as is the magazine release and spring. Even the barrel and takedown lever are cross compatible too. And yes, the backstraps are, as well!

It even appears that the ejectors, and recoil rods are compatible with each other too.

That's where the cross-compatibility ends though. After that point, it's mainly aesthetic similarities- such as the profile of the slides and frame.

Design of The Frame

When I first looked at this kit, I thought to myself "this looks doable!", namely because of the rail sections. However, this kit was much more tedious to work on than expected.

My first steps were to take accurate measurements of the pressure/force-bearing parts, namely the rail plates. This was a tedious hassle, just because of the number of rail pieces and odd, slanted profile of them. After that, I had to "assemble" the parts together in my editor.

Following that, I measured the hammer housing. The hammer housing was difficult, as it had many complex curves, so my first few fitment prints of the rear frame had the housing sit at a slant. In fact, the first print I had with the slide on ended up having the slide binding because the housing wasn't perfectly parallel to the frame. Using my carving knife, I began to shave off pieces of the space where the housing sat, and adjusted accordingly in my editor.

Aligning the parts was a little tricky, but I was helped slightly with the FNS frame. I derived the magwell with its angle, magazine release geometry, trigger opening, and slide release location from the FNS frame that TMS had put together. That was a major time saver, and I was lucky to discover that the trigger pin is in the same location for the FNX as it is for the FNS too. I aligned the trigger pin hole accordingly with the rear hammer housing, and carved out enough space for the trigger arm-cage to move in the frame.

After that, I began my favorite part of designing these frames- the sculpting. I took a picture of an original FNX, aligned the pin holes as best as I could, and then began to draw the 2D profile of the frame first.

I always try to sculpt the trigger guard first. I think this is the most important part of the ergonomics right before the backstrap. Using choice tangents and ellipses, I made the trigger guard look as similar as possible to the original frame. In the "deburring" process of the design, where I take out sharp edges, this shape becomes curved and comfortable to hold.

While it's not easy (or original!) to replicate the original frame, I did try to overcome this by adding my own spin on the FNX design. Namely, I kept the grip profile straight like a 1911, and extended the beavertail slightly to somewhat encapsulate the hammer. In addition, the "foot" of the magwell is more pronounced.

After a week of printing and measuring, I reflected any changes that needed to be made (such as the slide release binding by being ever-so-slightly longer) into the editor for a well-dialed in design. I wish I could get to a range or to my firing trap downstairs to test the gun, but alas, I am confined to my bedroom for now.

Overall, I'm very excited I finally got this thing put together. This has been in my backlog since it arrived from the generous donor, and it's one more kit that can be put together.

The only unfortunate part is the scarcity of the parts. My market data shows that no kits have been spotted for at least six months, and EGP only has three kits cached on search results.

Cheers,

Vinh