Build Recipe 5 - Clay's Sig MPX Truck Sub Gun

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There are so many ways you can outfit a Sig MPX. The customizability is probably one of the best parts of owning an MPX. The aftermarket support is strong with several barrel length options, hand guards, folding and telescoping stocks, triggers, safety switches, grips, and more.

When I set out to build a gun that would fit in the center console of my truck, I had a list of criteria I wanted the build to meet

  1. I wanted it to be able to functionally engage targets out to 100 meters, which means a folding or telescoping stock, and a reddot optic.
  2. I wanted it to be suppressed so it could be shot from within the truck if needed, without hearing protection
  3. I wanted it to have an adjustable gas block to dial the gas in for shooting subsonic rounds suppressed.
  4. I wanted it to be able to shoot in its folded or collapsed stock position if necessary
  5. I wanted it to have a functional sling.
  6. I wanted it to have at least 30 rounds of magazine capacity.
  7. and I wanted it to be fun to shoot.

Step 1: Finding a host

Build Recipe 5 - Clay's Sig MPX Truck Sub Gun

I started with a 9mm Sig MPX-K Pistol that comes with no stock. Because my distance goal is within 100 meters and sound suppression is a high factoring criteria for this build, 9mm felt like a reasonable choice. I can use readily available 147gr subsonic ammunition that is available in both self defense hollow tips as well as less expensive full metal jacket ball ammo for training.

Thereason I started with this form of an MPX is because no tax stamp is required to buy the gun in this form and it can be taken home and shot in pistol form while waiting for an SBR tax stamp to come from the ATF which would allow a stock to then be added. 

The current generation of MPX-K pistol comes with a 35 round magazine and a Timney single stage trigger. The closed bolt gas piston system in the MPX allows it to be shot without a buffer tube or stock, making it an excellent host for a folding gun that can be shot while folded.

And the 4.5" Barrel is short enough that the over all length of the gun will still be short once the suppressor is added.

Step 2: Picking out a Stock

Build Recipe 5 - Clay's Sig MPX Truck Sub Gun

Sig offers a number of different stocks that all do the job. But ever since I saw the JMAC Customs folding Skeletonized aluminum riser stock with 1913 hinge, I knew that was what I wanted. It's light weight and has a rubberized pad on the back side of the stock. There are optional cheek pads sold separately, but I prefer mine without the additional height.


Step 3: Picking out a suppressor

Build Recipe 5 - Clay's Sig MPX Truck Sub Gun

The MPX-K comes with a removable muzzle device that reveals an M13.5x1 Left Hand threaded 4.5" barrel. This is a relatively uncommon thread pitch making it slightly more complicated to find a suppressor that threads on directly. 

There are adapters and various muzzle devices that convert from 13.5x1 LH to something more common like 1/2x28, Keymo, or Trilug, but I didn't want to add overall length by using an adapter.

Another option would have been to swap the entire barrel out for an aftermarket barrel that is threaded with a more common thread pitch. But I would rather avoid that additional cost unless I was changing the barrel length as well. But for this gun I wanted to keep it as short as possible.

I already owned a Dead Air Wolfman 9mm suppressor that ended up feeling a little too girthy for my preference on a my Staccato P and I saw that Dead Air actually makes a 13.5x1 LH direct thread end cap so I removed the Sig muzzle device and threaded the Wolfman on with the 13.5x1 LH end cap and it looked and felt great! I am running the Wolfman in its shorter configuration. 

One thingworth mentioning here. The Dead Air end-cap threads into the suppressor with Right Hand thread. And then the whole suppressor threads onto the barrel using left hand thread. if you try to thread it all together with your hands at the same time, you may crack one thread loose while trying to tighten the other. 

The best way to assemble this is to torque the end cap into the suppressor, then torque the suppressor onto the barrel by turning the end cap with a wrench, not by grabbing the suppressor body itself.

Formy personal build, I am dedicating this suppressor to this gun. Once it's installed, it will not be coming back off. So a direct thread end cap works for me. If you plan on removing your suppressor often to swap it to other guns or make the overall length of the gun smaller when stowed, you may want to consider a quick detach adapter and or an aftermarket tri-lug barrel.

Note: The large diameter of the Dead Air Wolfman ended up being a very tight fit inside of the Lancer hand guard. I actually had to install the hand guard and vertical grip in the hand guard and trim the excess threads off of the m-lok screws holding them on till they were flush with the m-lok washer just to make enough room for the Deadair Wolfman to fit into the hand guard and thread onto the barrel. If I were going to do this build all over again, I would use the Sig Mod9 titanium 9mm suppressor for its thinner diameter and adjustable length.


Step 4: Setting up the front of the gun

Build Recipe 5 - Clay's Sig MPX Truck Sub Gun

I am just over 6ft tall and when I shoot I tend to reach my support arm out towards the front of the barrel to keep it aimed on target when shooting fast. The MPX-K has a very short hand guard which makes it great for stowability, but can make it awkward to shoot for people with longer arms. Lancer Systems makes a lightweight carbon fiber hand guard in various lengths. I decided to go with an 8" hand guard and added a Sig Vertical grip to use as a hand stop. 

I went with a Surefire M340C Scout Pro 500 lumen weapon light with a Surefire dual tape switch. The flat part of the switch provides momentary activation while the circular button provides constant on/off. I usually use the Surefire M640U weapon light on rifle builds, but since I was trying to keep things small on this gun, I went with the single-battery version for it's size.


Step 5: Picking out an optic

Build Recipe 5 - Clay's Sig MPX Truck Sub Gun

Because this gun is designed for close quarters targets, a 1x red dot optic is more than adequate for the job, and a magnifier wont be necessary. My 3 personal favorite 1x red dots have historically been the Eotech EXPS, Holosun 510c, and Trijicon MRO (edit 5/29/2023: and now the Sig Romeo3XL). Originally for this build I decided on the Eotech because I really like the height, size, and shape of the viewing window. But I think I would have been just as happy with the 510C or the MRO.

EDIT (5/29/2023) - I recently made the switch from the EOTECH to the Sig Romeo3XL with the Sig riser and I have to say I like it just as much as the eotech, but with much less weight and less bulk obstructing the sight picture. The Romeo3XL glass is big at 35mm and bezel around the glass is thin and easy for my eye to ignore. I get a full sight picture without obstruction and the reticle is a nice simple red dot without a circle around it.

Step 6: Charging handle upgrade

Once you get used to using Radian Raptor charging handles, you really start to become overly sensitive to small charging handles. And the charging handle that comes on the MPX from Sig is fairly small in comparison. I replaced mine with a Radian Raptor-LT charging handle and it immediately felt better and more familiar when in action.


Step 7: Picking out a sling

The Sig MPX-K comes with 1 QD socket located at the top of the back of the pistol grip. This makes it a perfect location for a single point sling, which is perfect for a gun like this. My goal was to wear this gun with a single point sling, but I didn't like the idea of buying a dedicated single point sling for it. I wanted a sling that could go back and forth between single point or dual point so I can use the same sling with other guns that might not be ideal for single point slings. 

I ended up starting with a Blue Force Gear Vickers Sling because I really like how easy it is to adjust the length quickly between shooting or moving. The Vickers sling comes with no ends on it, so you can pick the correct ends for your build. I ended up picking the Blue Force Gear Rapid Engagement Detachment QD ends. They are like traditional QD ends but they have a knob that makes it easy to detach them quickly even when wearing gloves. Then I added a BlueForce Gear Burnsed Socket that allows you to convert the sling from 2 point to 1 point by connecting the sling to itself via the burnsed socket.

The final product is a premium quality Blue Force Gear Vickers sling with upgraded RED QD ends and a Burnsed socket that allows conversion from 2 point to 1 point configuration on various different guns.