SILENCER SATURDAY #226: The MCX Virtus Piston System

by Pete
SILENCER SATURDAY #226: The MCX Virtus Piston System

Good morning everyone and welcome back to TFB’s Silencer Saturday brought to you by Yankee Hill Machine, manufacturers of the YHM R9 multi-caliber rifle suppressor. Last week we got another look at the awesome B&T STATION SIX9 pistol and the seriously quiet wiped suppressor. This week we take a look at the MCX Virtus piston system and the SLH300TI-QD. How does this arranged marriage work out? Let’s take a look.

SIG MCX Virtus @ TFB

I am going to consider this Silencer Saturday installment a part of TFB’s Ultimate Guide to the MCX platform. These SIG’s are among my favorite guns of all time. Mostly because they are built to be suppressed, are easy to operate, and are very compact. But if you are only used to the AR-15 platform, there are a few things we should go over – because the MCX is not an AR.

SILENCER SATURDAY #226: The MCX Virtus Piston System

I. Introduction

If we want to go back to the very basics, all semiautomatic/automatic firearms require force to cycle the action to eject a spent casing and load a new round into the chamber for firing. In the AR-15, this is called a direct impingement (DI) system; gas from the last round fired follows a path from the barrel, down a gas tube, and into the action where the bolt carrier group is pushed rearward to cycle the action.

The SIG MCX Virtus operates on a short stoke piston system. Gas from the last round fired pushes a piston which pushes an operating operating rod on the bolt carrier group which in turn cycles the action. The main benefit over a DI system is that, because the gas never makes it to the rifle’s action, it runs cleaner. The SIG MCX adds the benefit of a two position gas adjustment that controls the flow of gas into the piston is that the gas port size (the pathway from the barrel into the piston) can be changed based on ammunition quality or suppressor use.

The 6.75” barrel MCX was specifically built to use a suppressor, so I treat the gas adjustment as a choice between running supersonic ammo or subsonic ammo. In other MCX barrel lengths, the gas adjustment can also be used to combat weak ammo or a dirty and un-lubricated action. The 6.75” barrel was designed around the now discontinued line of SIG SRD suppressors. The latest SLH suppressors are a completely different design from the previous generation of silencers, meaning the amount of gas that passes through the gas port and into the piston is going to be different. When we tested the new SLH300TI-QD on the 6.75” MCX barrel, neither gas position would properly cycle subsonic ammunition.

I reached out to SIG and let them know about my observations and they sent out a new piston assembly. Let’s take a look at what that means.

II. The parts of the MCX short stroke gas piston system
SILENCER SATURDAY #226: The MCX Virtus Piston System

Neither of the above barrels are the 6.75” Virtus barrel; the top is the 5.5” 300BLK Rattler barrel and the bottom barrel appears to be The 11.5” 5.56 Virtus barrel. Because the length of the barrel directly effects the dwell time (the amount of time the bullet is in the barrel to funnel gas into the system), the gas port sizes are different to prevent over or under gassed issues.

The standard MCX gas piston assembly for the 6.75” 300BLK barrel is marked with F01 ( SIG Store).

SILENCER SATURDAY #226: The MCX Virtus Piston System

The standard MCX gas piston assembly for the 9” 300BLK barrel is marked with F02 ( SIG Store).

SILENCER SATURDAY #226: The MCX Virtus Piston System

For comparison, the standard MCX gas piston assembly for the 11.5” 5.56 barrel is marked with J02 ( SIG Store).

SILENCER SATURDAY #226: The MCX Virtus Piston System

To address my cycling issues with the SLH300TI-QD on the 6.75” barrel with subsonic ammunition, SIG sent over a new piston assembly specifically for this combination. It is marked as ‘KIT, PISTON ASSY, 6.75 IN BARREL, 300BLK, SLH300 SUPPRESSED’ with a part number of 8900727. It is not yet listed on SIG’s online store. It is marked as F03.

SILENCER SATURDAY #226: The MCX Virtus Piston System

I don’t have a drill rod set to measure the gas port sized, but the F03 assembly has larger gas ports than the original F01. Below is the F03 on the left and the F01 standard assembly on the right.

SILENCER SATURDAY #226: The MCX Virtus Piston System
III. Replacing the MCX piston assembly

Removal of the MCX piston assembly is straight forward, but I have to recommend that you utilize a certified MCX armorer for all modifications past the basic field strip cleaning and inspection process.

While depressing a small plunger on the front of the gas block, rotate the piston assembly clockwise and remove it from the block. The piston itself can be removed by using a brass or polymer punch to push it out from where the adjustment knob was seated.

SILENCER SATURDAY #226: The MCX Virtus Piston System}

Reinstallation is completed by reversing the above steps. I used a small amount of high temp anti-seize to seat the piston smoothly.

SILENCER SATURDAY #226: The MCX Virtus Piston System

Reinstall the Clutch-LOK muzzle device ( SIG Store).

SILENCER SATURDAY #226: The MCX Virtus Piston System

Properly attach and lock the SLH300TI-QD and reinstall the hand guard.

  • Model: SLH300TI-QD
  • Manufacturer’s Page: 
  • Caliber: 300 Blk
  • Material:  Titanium
  • Diameter: 1.67 in (42 mm)
  • Length: 8.8 in (224 mm)
  • Weight: 1.0 lbs (16 oz)
  • Mounting System: Clutch-Lok QD
SILENCER SATURDAY #226: The MCX Virtus Piston System
IV. Observations

With the F03 marked piston assembly for the SLH300 installed, the 6.75” MCX operated flawlessly with the new suppressor. The (-) setting cycled supersonic ammunition perfectly, with a 90 degree ejection pattern, and no noticeable gas blowback. On the (+) setting, subsonic ammo also performed as expected. I still consider the SLH300 to have top-tier performance when compared to the SRD762TI, which is is one of my all-time favorite 300BLK suppressors. And the MCX is my favorite 300BLK platform.

Critics may want to point fingers at SIG for not anticipating the need for a piston assembly swap to run the SLH300 line of silencers. I would remind them that the SRD models were developed and brought to market nearly ten years ago and that the push for low toxicity for operators with new suppressors has been a more recent development. My assumption is that, since the SRD line is discontinued and the SLH models are SIG’s 300BLK flagship, the F03 piston will be standard in the new 6.75” MCX guns going forward. Which means no piston swap will be necessary.

Next week we’ll bring you the first in a three part series on the AB Suppressors 5.56 and 7.62 Raptor models. Thanks for reading,be safe, and we’ll see you back here next weekend for another Silencer Saturday.

SILENCER SATURDAY #226: The MCX Virtus Piston System

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4 of 22 comments
  • Mig1nc Mig1nc on Apr 30, 2022

    Not enough FDE on that gun. Pete needs pick up the Sig tele-folder in FDE to complete the package.

    • Suppressed Suppressed on Apr 30, 2022

      @mig1nc Good luck finding one, I’m having my black one cerakoted to match my AK.

  • LazyReader LazyReader on May 01, 2022

    Isn't MCX already piston?
    recalled, rebuilt, and resold.


    comment photo
    • H538 H538 on May 01, 2022

      @LazyReader 3 now. The Legacy, the Virtus and now the Gen 3. I regretfully bought into Sig thinking they would stay with the Virtus for a bit. I should have listened to everyone online saying what a shit company they are because they are. They come out with a product, advertising how much stuff they are going to make for it, none of it is ever in stock. Before it comes in stock again it’s discontinued and they are onto a new Gen that’s not compatible with the last. Take my advice, get or build yourself a top tier mid-length AR and prosper.