JSD Supply MUP-1, Build and Review

Source: JSD Supply

After completing my first 80% AR-15 and Polymer80 builds, my thirst for more DIY gun projects only grew deeper. The prospect of an 80% Sig Sauer P320 greatly piqued my interest, but I was disheartened to see them either out of stock or as unreleased vaporware everywhere I checked. Turns out I didn’t look hard enough. November of 2018 I went to the Split Rock Gun Show in Lake Harmony, PA. After browsing through the rows of booths and making a couple of purchases, I was pleasantly surprised by what I saw; I found my white whale, sold by a company I’d never heard of before. Enter JSD Supply and their MUP-1 (Modular Universal Pistol) 80% fire control unit (FCU).

The man running the booth was incredibly helpful, going over every build kit and conversion kit he offered in detail, and demonstrating that their 80% FCU had been updated to match the P320’s drop safety upgrades. Being a student at the time I could only afford the 80% FCU and the drilling/bending jig with included drill bits. The project ended up sitting on the back burner for almost a year, with school, exams, and a new job consuming most of my time. 

When I eventually ordered the FCU parts kit and a full-size grip module and slide assembly (referred to as an X-Change kit in Sig nomenclature), I was once again surprised. The X-Change kit came in a Sig Sauer branded hard case, and my parts kit came with an Action Enhancement Trigger from Apex Tactical, as opposed to the OEM curved or X-Series flat trigger from Sig Sauer. Whether intentional or by mistake, I welcomed this new addition, and the build could once again continue.

The Build Begins

On a slow day where I knew nothing would be done, I finished the FCU. Using the jig’s top plate and a hammer, bending the FCU rails was a simple task, finished with a squeeze in a bench vice to ensure the four rails were evenly bent to spec. Drilling speeds for the variety of differently sized bits were gleaned from The Rogue Banshee, and the whole drilling process went without a hitch. I’d say his “build adventure” / tutorial series is the best one out there if you’re looking to complete a MUP-1 for yourself; it’s detailed, easy to follow, and covers most questions and concerns that potential builders may have.

Forming the takedown lever slot at the rear of the FCU.

Once the FCU rails were bent and all the holes were drilled, I got to work cutting the slide rails to length. I didn’t have the sharpest of hacksaw blades, so while the rails were cut to length along the top plate, they had humongous burrs and the forward rails had this kinda down-and-inward positive camber cut to them. Opening up the hole on the right side to accept the trigger was easy using a Dremel with a carbide bit and a file to finish, following the contours of the jig. On the other hand, opening up the slot for the takedown safety was a pretty tedious task, as I didn’t have the right size Dremel bit and opted to use a set of needle files. All that was left was to size the FCU rails to the slide.

I was worried that the forward rails were messed up due to how they were cut, and that filing down all the rails enough to completely deburr them would lead to undersizing. These worries would disappear after taking measurements. Using The Rogue Banshee’s rail calculator as a general guide, the rails were filed down and the burrs were cleaned up with room to spare. I carefully shaved down each side until the FCU was just able to fit in the slide rails without excessive force, then I filed down any high spots I detected with the use of layout dye. After rounding out the sharp corners with some emery cloth the fit between the FCU and the slide was butter smooth with negligible play. Now that the “gun” was legally considered complete, it was time to make it work.

Installing the parts kit was straightforward thanks to The Rogue Banshee’s FCU parts installation tutorial, though I did encounter one major hangup in the form of the safety lever and safety lever spring. At some point in time, the engineers over at Sig redesigned the safety lever and determined the spring was unnecessary. Alas, my parts kit was older and required the spring be installed. The only problem was that The Rogue Banshee’s assembly video, while it did show installation of the spring, it didn’t exactly specify where the legs of the springs needed to rest in the safety lever housing. It was only after a little bit of Google-Fu that I found a forum post detailing the proper position of the spring legs. I used a flat-bladed eyeglass screwdriver to move the spring legs into place (check the photo above!), and sailing was smooth from that point forward.

Inserting the FCU into the grip module was simple enough, and inserting the takedown lever itself was a non-issue. However, the takedown lever was incredibly stiff to turn. This issue isn’t uncommon from what I heard so I proceeded with a basic function check with and without snap-caps. After passing all these function checks the takedown lever was still stiff, but after field-stripping the gun another 10 or so times, the rubber o-ring on the takedown lever wore in and the lever became much easier to manipulate. Fire testing was wholly uneventful, the gun eating boxes of 115gr Fiocchi for lunch with no failures. Having never shot a Sig pistol before, recoil was actually a little less than I expected given all the moaning about high bore axis. Then again my experience with pistols is sorely lacking. After fire testing I eventually replaced the old grip module with a full-size X-Series grip module that better fit my hand.

Final Thoughts

If you’re looking for a P320 review you’ll have to look elsewhere because man do I suck at reviewing pistols. As of writing this article, the MUP-1/P320 is only the second pistol I own after my Polymer80 PF940C/G19 build and as stated prior, my pistol shooting experience is very limited. Now that that’s out of the way, I’m very satisfied with the MUP-1. The build itself is very straight forward, and the only power tool that’s absolutely required is a drill press with an X-Y vise. We’re working with metal, so the completion process does require precision and patience, over say a Polymer80 frame that can be completed in under an hour with basic hand tools. My build start-to-finish took around 5 hours: 4 hours at work finishing the FCU (with some breaks), and about 1 hour at home installing the parts kit, assembling the finished pistol and function testing. But as satisfied as I am with this gun, we have to talk about the elephant in the room: the cost. 

The MUP-1 is expensive, especially in comparison to the Polymer80. Now, JSD Supply is a very small company supplying a pretty niche product (as far as I know they’re the only producer of 80% P320s; other websites resell JSD product), and supply/demand and economies of scale are a thing and all that but man the cost is a painful pill to swallow. A complete build kit for a Polymer80 PF940v2/G17, a comparable full-size handgun, goes for $500 over at Rockey Brass. My MUP-1 on the other hand cost $880. The FCU and jig were $250, the FCU parts kit was $230, and the X-Change kit was $400.

So why did I build the MUP-1 when building a Polymer80 or buying a P320 at a gun store is so much cheaper? Well, to put it simply, I wanted another handgun that wasn’t a Polymer80/Glock, and with my home state having draconian firearms laws and an absolutely byzantine pistol licensing procedure, an 80% is the quickest way I can currently get a pistol, while still remaining within the bounds of federal law. The fact that I don’t have to go to an FFL, and the fact that it isn’t serialized is an added bonus. I’m also very interested in how the aftermarket for the P320 will develop as more and more companies see the potential of the platform. With how modular it is, you can extract the FCU from whatever frame you put it in, and use it as the trigger group for something that isn’t a handgun. Whether it’s a PDW like the Flux Defense MP17 or the upcoming Strike Industries grip module, slapped in a rifle-caliber firearm like the (hopefully) upcoming X03, or used for something wildly different, the possibilities are definitely out there.

Is it worth it? Much like my GG2 review, yes and no. If you just want a gun, there are cheaper and easier ways to go about it. But if you’re someone like me who lives in a place with restrictive laws, likes to build and tinker, supports small businesses, and are interested in the P320, I’d say go for it. There are much worse things to spend your money on. Like Remingtons. Or Kimbers. Eugh.


Writer with a focus on emergent technologies in arms manufacture. Currently trapped in no man's land.

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2 Responses

  1. Good article and thanks for the shout out and kind words on our build video of the P320. Congrats on your build! Glad it went well.

  2. Mark McLane says:

    Great write up.

    I have recently finished my first MUP 1 build. 320 Carry with a build kIt from JSD and their jig. I really appreciate what they are doing out there. The cost was about the same. I too enjoy the hands on and the education that is part if the process.

    I have also watched The Rogue Banshee and appreciate those tips.

    I had some fine tuning of the FCU in the grip frame.

    When I cycled the trigger I observed that the the FCU rode up at the rear where the disconect block sits. It moved up when trigger pressure went up as I followed through to disconnect. It lifted about 10-15 thousanths, .010-.015. I could see the movement. I think this made the trigger the tiniest bit spungy. The tail piece on the FCU that fits into the slot in the back of the grip frame needed a tiny bend to wedge it into the slot in the grip frame. After I made this adjustment there was no longer any movement and the trigger hardened up with a consistant 6lb disconnect.

    Thank you for sharing your experience.
    I recommend this project.

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