Sig Sauer MCX and KeyMod System

Sig Sauer MCX and KeyMod System

Making a change to accessories

By Dennis Adler

The Sig Sauer MCX airguns have very similar lines and use the same lightweight KeyMod-style handguards as the centerfire models. KeyMod handguards are designed with different inner diameters so that they can fit over barrels or sound suppressors as needed. The longer handguard design (as seen on the MCX) acts as a continuation of the one-piece top rail, giving you over 15 inches of accessory rail on the CO2 models. The KeyMod handguards will also accept a variety of accessories including the vertical foregrip that comes with the CO2 model, as well as mounting tactical lights and lasers. The CO2 model shown is fitted with the actual Sig Sauer Bravo 4 Battle Sight used on the centerfire models. Also pictured are a Sig Sauer P226 CO2 pistol in FDE (holstered) and a Surefire Luminox 2211 tactical light combat watch.

It’s Thanksgiving and so this is going to be a short read, but one that answers a very good question about the Sig Sauer MCX CO2 semi-auto tactical rifle. It has a foregrip that comes mounted to the handguard and for some it is positioned too far back on the handguard rail. I found the factory mounting position good because it is far enough back that it helps push the support arm into the body for a more solid hold. But if you have longer arms it could be too close.

Both the centerfire and CO2 pellet models use a KeyMod accessory handguard which makes fitting of accessories simple and quick to mount. It comes from the factory with the foregrip positioned to the rear and closer to the magwell as shown.

The centerfire Sig Sauer MCX model uses a KeyMod system handguard which is designed for the rapid mounting and dismounting of accessories and accessory rails. Developed around 5-years ago, the KeyMod looks like a series of keyholes into which any accessory with a machined mounting screw and nut or (two or more) can attach and be solidly locked into place. This eliminates the need for full length Picatinny rails on rifles like the MCX and AR-based designs, as shorter rails can be mounted wherever needed on the top, sides or bottom in seconds, or an individual component, like a foregrip can be easily attached.

The CO2 model has a functional KeyMod design and the foregrip can be removed using a Phillips head screwdriver. Actual KeyMod accessories, including tactical foregrips, are mounted the same way but use military grade hex head screws.

The Sig Sauer MCX CO2 model uses a real KeyMod design handguard and a foregrip that locks into place using Phillips head screws (rather than hex head screws used on actual KeyMod accessories) but works in the same way. At first when the question was asked, I doubted that the CO2 model had an actual working KeyMod foregrip because of the different screws (Phillips head) and the look of the nut inside the handguard. But, in fact, it works the same and you can move the foregrip by slightly loosening the two Phillips head screws so that the nuts at the top of foregrip can slide back and out of the KeyMod slot, allowing the foregrip to be removed by pulling it straight down and out. Then just place it where you want it, push the nuts up into the large keyhole slots, slide it forward into the narrower slot and screw it down. All very simple if you are familiar with the KeyMod system and go with the assumption that the foregrip on the CO2 model can be removed the same way. After a quick call to Sig I found out that it could and then followed the same process as any KeyMod handguard and accessory.

The foregrip on the CO2 model is removed by slightly loosening the two screws to relieve tension on the locking nuts, sliding the grip back to position the locking nuts in the large keyhole, and then pulling the foregrip off the handguard.

Here the locking nuts can be easily seen. They slide into the keyholes wherever you want the grip positioned; the grip is then pushed forward so the nuts slide into the small slots, and then the screws are tightened to lock it in place. That’s the KeyMod system.

Here I have repositioned the foregrip to its optimum forward position for those who have longer arms or prefer the grips further away from the magwell. It can be moved anywhere in between. However, there is one little bit of bad news about moving the foregrip. The MCX comes in a spectacular fitted box, which makes a great way to store it, unless you want to put it in a gun cabinet or gun safe. The interior of the box is form fitted to the MCX with the foregrip in the rearward position, so if you move it, the MCX won’t fit back in the box.

If you don’t have the Sig Sauer MCX CO2 model, this probably isn’t all that helpful, but it might also seal the deal if you’re on the fence. This is one powerful (88 gram) CO2 powered, pellet-firing air rifle and it can be equipped as shown or with any accessory that fits on a centerfire model. It is a practical tactical trainer and will give you a feel for the real gun and using the KeyMod system to mount accessories.

The MCX CO2 model with the foregrip moved further out will work better for some operators.

Happy Thanksgiving! This Saturday we take the two most interesting CO2 pellet cartridge firing WWI era revolvers, and shoot them head-to-head. It’s going to be one very revealing airgun test!

2 thoughts on “Sig Sauer MCX and KeyMod System

  1. Thanks Dennis for getting an answer about the foregrip. I just looked and failed to find any KeyMod accessory rails available for the MCX at Pyramyd Air. Do you know of any rails that may be available?

    After reading some customer reviews reporting CO2 leaks and pellet jams, I was a bit anxious about buying the MCX. However, it performed very well for me yesterday without any evident CO2 leaks, pellet jams, or any other problems.

    Like some others, I was curious to determine if a 2 x 12 g CO2 adapter could be used. I have two such adapters, one for the Umarex Hammerli 850 rifle and another for the Umarex Walther Lever Action rifle. The Sig Sauer MCX requires a minimum stem length of at least 1 inch on the CO2 cartridge or adapter. The Air Venturi 90 g and the Umarex / Walther 88 g cartridges have stems that satisfy the minimum 1″ length requirement. I tested the fit with a couple of empty Air Venturi and Umarex / Walther cartridges before using a full cartridge. The two 2 x 12 g CO2 adapters however have stems that are significantly less than 1″ in length and will not seal in the MCX before the bodies of the adapters bump against the back of the MCX frame.

    Rather than using the open sights, I used a Center Point Multi-Tac dot sight (PY-A-3647). I shot a variety of pellets from a bench rested position at 10 meters (33 feet) distance to target. My smallest 10 shot groups were 0.75 inch using RWS Hobby, H&N Finale Match Rifle, and H&N Match Green Lead Free pellets. The 0.75 inch group with the Match Green pellets is particularly significant in that it represents shots 271 to 280 on the CO2 when I expected to be nearly out of CO2.

    I stopped after 310 shots. When I removed the Air Venturi CO2, the amount of residual gas that escaped made me think there may have been enough CO2 for possibly another 30 shots.

    The MCX proved to be a very good investment.


    • Sounds like you’re on target. As for KeyMod accessories yu can find them at most online sites that sell AR accessories like Bravo Company. KeyMod rails are not that expensive and will mount on the Sig. Then you can mount what you want on the rails. You might also find a better (or nicer) foregrip with KeyMod mounts. Enjoy the MCX!


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