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Review: Fire Control Unit X01, a Modular Pistol Chassis

This article originally appeared in CONCEALMENT Issue 12

KINDER EGG CARBINE: The Fire Control Unit X01 has a Surprise Inside

It all started with the SIG SAUER P250. First introduced in 2007, and then largely abandoned merely a few years later, the P250 was one of the first, if not the first modular chassis system released by a major manufacturer. A chassis system refers to a design in which the receiver (the serialized “gun” part of a gun) is separate from both the slide and the grip.

Ostensibly this means you could hot swap a chassis from grip to grip, from slide to slide, from caliber to caliber. But this wasn’t to be with the P250. SIG revised the grip in 2009, and previous magazines that worked no longer functioned with the new grip style. While some caliber and grip conversions exist, the platform never took hold like SIG originally intended. Ultimately we’d call the P250 a commercial failure, but a step in the right direction for modular chassis.

The idea of how modular a SIG P250 could be inspired the owners and engineers of Fire Control Unit to start daydreaming. Sure, grips and calibers are nice — by why not go further? After many design iterations, and the availability of consumer 3D printers, FCU was born. Just five years earlier this venture would’ve required an investment in the seven-figure range, only available to large companies. But now? Manufacturing is on its head. If you peruse the FCU webpage, you can see that all of the first models were 3D printed, though the production models are injection molded. Some small parts like the charging handle and grip stops are still currently made on rapid prototyping machines.

Compact slide with a fullsize barrel certainly looks funky in pistol form

Compact slide with a full size barrel certainly looks funky in pistol form


Functionally, what separates the FCU X01 from other pistol-to-carbine conversions is that a complete pistol isn’t needed to insert inside a frame à la the CAA Roni, Hera Arms Triarii, or FAB Defense KPOS.

With that said, you do currently need a P320 slide, chassis, and barrel (at least for now — more on this later). The X01 is designed to accept a SIG MCX collapsible stock or brace, though you can order one without. Any stock that can attach to a Picatinny rail will work, and though we’re not saying we modified a Micro Uzi stock to fit one, we were able to modify a Micro Uzi stock to fit the FCU.

Admittedly, the complete X01 looks a skosh like a Beretta Storm and a SIG MCX had a weird baby. But it’s far more modular than either. The X01 is a two-piece design consisting of the lower frame and stock, and the upper receiver with a continuous Picatinny rail and push-button rear latch. The front takedown pin features a neat captive pin (we broke one, of course. But a replacement was sent out the same day).

If your trigger doesn’t reset after assembly, push forward on the trigger bar tab.

If your trigger doesn’t reset after assembly, push forward on the trigger bar tab.

There are two integral QD sockets where the stock attaches to the X01, opening up a plethora of support options that aren’t available with a stock P320. The X01 is compatible with full-size, compact, carry, tacops, or X-Five P320 and P250 firearms chambered in 9mm, .40, or .357 SIG (the last if you’re weird AF).

What’s in the Box

The package we received from FCU included the chassis itself, a collapsible brace, instruction manual, pre-cut grip tape, and some goodies from Springer Precision including a magazine extension and an extended magazine release.

First, we must admit that we lost the grip tape. It’s probably somewhere behind a cabinet, along with a couple socks and buffer retaining pins. Later, we cut some 3M ladder tape which served its purpose well. Hooray for Harbor Freight heat guns making the process easier.

Normally, we’d just dive right in without reading the manual, but in this case we implore you to read the manual. There’s a continuously updated manual online as well, to incorporate revisions and lessons learned in the future.

Conversion Assembly

Fire Control Unit has a video showing you can swap everything over from a pistol to a carbine in under 20 seconds, though they don’t advocate performing the switch in the field. In reality, it’ll take you some more time, so please don’t imagine you can convert your duty gun while on the scene of an active shooter scenario.

The largest pain in the ass is the fault of SIG SAUER and not FCU. While it’s easy as hell to pull out the SIG chassis compared to other chassis guns like the Beretta APX, it’s the magazine release we found the most troublesome.

Adjustable recoil spring assemblies make shooting the X01 suppressed better — at least after some tinkering.

Adjustable recoil spring assemblies make shooting the X01 suppressed better — at least after some tinkering.

Removal of the P320 magazine release requires the use of a paper clip or other small item to depress a plastic tab for disassembly and removal. Why everyone doesn’t just go the route of the 1911 and only require a screwdriver is beyond us. Either way, if you purchase an FCU, do yourself a favor and just buy a second complete mag release for dedicated carbine use.

You may run into another quirk of conversion — a dead trigger. In order to fix this, pop up your top cover, lock your slide to the rear, and push forward on the trigger bar tab. After that, it should be smooth moving. If your trigger isn’t resetting on your X01, this is probably the culprit. We’ve gone so far as to include this as part of our assembly process, since it was a problem that regularly plagued us.

Configuring & Controls

Given the closet-quarters nature of a short-barrel pistol-caliber carbine, we felt A-OK popping an EOTech right on top, though any red dot will find a happy home on the FCU X01. Though real estate is certainly at a premium, we knew we’d need a white light. While the new 1,000 lumen SureFire X300U certainly fits the bill, we found the bayonet-type mount to be less secure than the thumbscrew B-models. The 1,000-lumen SureFire XH35 found a home, working well within the performance envelope we desired.

There’s an M-LOK channel in the bottom of the X01, but we couldn’t put a stubby vert grip there because it would make this pistol an illegal SBR. Those outside the United States won’t have such worries. Regardless, the standard grip and accentuated handstop make it easy to activate the light from either shoulder with no undue risk.

We're sure not even SIG SAUER considered all the possibilities... but the aftermarket has.

We're sure not even SIG SAUER considered all the possibilities… but the aftermarket has.

In the standard configuration, the X01 has nearly ambidextrous charging handles. While either 3D printed nub can be used to rack in a round, with the Gen1 X01 only the left-hand side can be used to slap-down the bolt, HK style, or manually lock the slide to the rear. As far as manual safeties? Unless you’re using the FCU designed for the M17, no dice (and we don’t care either).


Our motto is to put a can where we can, so of course we wanted our FCU to be suppressor-ready. Unfortunately, you can’t take just any silencer, slide, and barrel combination and make it work. In order to run a silencer with the X01, you need a Compact/Carry chassis, P320 Compact slide, full-size threaded barrel, and a pistol suppressor and booster assembly that’s very small in diameter (1.3 inches is what we were told).


The suppressor/booster diameter matters for two real reasons, namely the limitations of the opening of the chassis itself and interference with the charging handle (the latter being the reason why you need to use a full-size barrel and a compact slide).

And then, you’ll still have some problems. We obtained a full-size P320 threaded barrel from SilencerCo; SIG not only threads their barrels in weirdo metric, they simply do not currently make a threaded barrel for their full-size pistols. This wasn’t an issue. Finding a silencer that worked? Well, that was a larger problem.

The first thing you need to do is configure this barrel/slide combo in a regular P320 setup. Yes, it’ll look weird as hell. Chances are, if you can find a suppressor with an appropriately meager diameter, it still won’t run very well. This is because the P320 uses a tilt-barrel action — the further forward the weight of a silencer, the harder it is to run. FCU recommended ensuring the booster assembly is impeccably clean and properly lubricated. We still ran into some issues but found a way to make it work.

Firstly, we ran the slimmest silencer we know: The Dead Air Odessa-9; despite some claims, we had size/charging handle clearance issues with nearly every other suppressor we tried. Even then we ran into problems until we started to use very heavy ammunition such as 158-grain Israeli ball and 165-grain HUSH from Freedom Munitions. Moreover, the Odessa-9 has to be configured as a different size for each load.


So we got ’80s with it. That is to say, we started playing mucky muck with the recoil spring assembly. While there are a couple of interchangeable-spring RSAs for the full-size (both Springer Precision and GrayGuns came highly recommended), much like the barrels, it was harder to source an assembly for the compact. To the internet we went, coming upon DPM
Systems Technology Ltd. This Greece-based company was the only source we could find. After waiting a couple weeks for our overseas package, we cut open the box to reveal a non-captive recoil spring assembly and three different springs to choose from.

At the end of the day, we still had to cut some springs like when Bruce Springsteen topped the charts, but we were about to make the FCU X01 run reliably with the Odessa-9 in the short configuration with ammunition that was at least 147 grains.

Persnickety. That’s what we’ll call running this firearm suppressed.

At the Range

Aside from our suppressor troubles, we had few issues at the range. The collapsible brace made for a compact package that was far more accurate and easier to shoot than its pistol version. The one issue we did have didn’t have anything to do with the X01 itself, but with the garbage ProMag stendo’ we obtained. Of course and as usual, it was a subpar product. It looks really cool though!

Unless you’re looking for likes on the ’gram, stick to OEM 21-rounders and extensions from companies worth a damn. We’re not saying they’re incapable of making a great product — we just haven’t seen one yet. Due to the SIG M17 being picked up by Big Army, you can expect a whole boatload of mag options to drop in the near future.


When the stock/brace is fully collapsed, the rods interject themselves into the same area as the ejection port, but we had no issues shooting with the stock in the smallest position.

The Future

Running a silencer would be far, far easier if the barrel were fixed. We’re told the next generation, the FCU X02, will feature a dedicated bolt and a fixed barrel. Instead of a chassis/slide/barrel combo, all you would need to add is the chassis itself.

But in the words of the now-deceased, coke-fueled Billy Mays, “But wait — there’s more!”
The 5.56 FCU AR320. Yes, you read that right. Coming down the pipe is a 5.56 AR platform that runs on … you guessed it: a SIG P320 chassis.

Loose Rounds

The FCU X01 is certainly a Gen1 system. There are many small changes and refinements that would make it a better carbine, but all isn’t lost — they’re very receptive to criticism and fast to implement changes; the advent of the internet and 3D printing means that they can be very responsive. We’re sure not even SIG SAUER’s engineers considered the possibilities when they designed their first chassis pistol, but the aftermarket has certainly answered.

Pardon us while we dream about a belt-fed version. And a select-fire chassis. Because reasons.



FCU evolution

The word “disruptive” is overused, especially in the technology sector, but Fire Control Unit is proof positive you don’t have to have $500,000 to make industry waves. This is just the first act of Fire Control Unit. The fixed barrel FCU X02 should eliminate most of the problems with running suppressed — provided that the opening is large enough. Theoretically, it’d be more accurate too. But what really got our juices flowing? A 5.56 AR that accepts a P320 chassis. While we don’t know all of the details (we should know more by the time this makes it to print), our best guess is that it will, at minimum, require a proprietary bolt carrier group.


So what’s the point when you can simply buy an AR lower for next to nothing at this time? Because we can, that’s why. We anticipate more chassis systems in the future, and already you don’t have to own a SIG P320 to play. There’s at least one 80% kit out there, and stripped pistol chassis are routinely sold for as little as $25 a pop.


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