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

• Ultimate Guide: The SIG Sauer MCX Rifle, Carbine And Pistol Platform
• SILENCER SATURDAY #215: SIG Sauer SLH300TI & SLH762 Suppressors
• Build an MCX – SIG Sauer’s SUR300 Suppressed Upper Receiver

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

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).

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

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

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.

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.

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.

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

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

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

Specifications:

• Model: SLH300TI-QD
• Manufacturer’s Page: https://www.sigsauer.com/slh-suppressor.html 
• MSRP: TBD
• 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

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.

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