Can we sticky this or something? The same questions come up all the time and are often miss represented. Please correct any mistakes I have made, and I will update the post.

Question: What parts interchange between LVAW, legacy, Virtus, Rattler, and Spear-LT platforms?

Answer: With the following exceptions all parts interchange between all versions of the MCX platform.
1.The generation of the upper receiver must match the handguard. That means that legacy receivers need legacy handguards, rattler receivers need rattler handguards, etc... True LVAW guns seen in military service have unique receivers that have longer receiver rails. Even longer than legacy. There are reproduction/clone LVAW handguards on the market for other receivers. Rattlers can only use Rattler handguards because their upper receiver rail is shorter than Legacy or Virtus. Some people have cut receiver rails or cut handguards to fit, but results may be of varying quality. Spear-LT handguards have a stabilizer screw in them that resolves earlier issues with rail flex. However, this screw and different rear profile means that some machine work is required to fit them onto a Virtus.
2.The barrel must match the bolt. Tapered lug barrel extensions will not lock up with non-tapered bolt lugs. This means you can take a Virtus barrel and bolt and put it in a legacy and vice-versa. The only issue is the cutout in the handguard to reach the gas adjuster will not line up in some cases.
3.Rattlers can use non-Rattler barrels with matching bolts if you also use the matching op-rod. And vice-versa.
4.There is very little difference if any between lower receivers except for the Spear-LT, which adds an ambi bolt catch/release lever.
5.You can use AR-15 receivers but only with the Sig receiver adapter. Non-Sig receiver adapters do not properly support the recoil spring assembly.
6.If you use an AR-15 receiver adapter, the Sig and Strike Industries MP5 style retractable stocks will not fully collapse due to the AR-15 receiver not being cut out for the rods. You can use the Maxim Defense telescoping PDW stock though since the rods ride lower.

Question: Can I use an MCX upper on an AR-15 lower?

Answer: Yes, but you will need to plug the rear of the receiver and only the Sig lower receiver adapter will properly support the recoil spring assembly since the MCX upper is taller than an AR-15 upper. See #5 and #6 above.


Question: Will my AR-15 trigger work in an MCX?

Answer: Maybe! Before the original recall (what I am calling "gen-2" below) any AR-15 fire control group (FCG) would work. During testing Sig discovered that some .300blk ammunition was loaded using primers that were sensitive to slam-fires. Therefore they added a firing pin safety spring to provide rearward pressure on the firing pin as well as a safety latch which prevented the firing pin from moving unless the latch was pushed out of the way by the hammer. This introduced two problems.

1. Some hammers did not have the correct profile to push the firing pin safety latch out of the way and would thus not strike the firing pin to initiate the round. Three different safety latches exist. One marked "S" (the default one) which only works with milspec style hammers such as the one in the Sig Match Lite Duo trigger kit. One marked "A" for M16 hammers. And one marked "G" designed for Geiselle FCGs.


2. The firing pin safety spring provides rearward pressure on the firing pin which requires more force than usual to overcome and initiate the round. Therefore very light hammer springs may not provide reliable ignition, resulting in light strikes. This, of course happens on the AR-15 too, but the MCX is more sensitive to it.

Some people have removed the firing pin safety latch and firing pin safety spring. Essentially returning to Gen-1 configuration. This is not recommended by Sig.

The MCX Spear-LT resolved this issue and AR-15 triggers are once again compatible. Keep in mind that 7.62x39 ammunition may require a stronger than normal hammer spring.

Question: What is the deal with this tapered muzzle / Why don't my muzzle threads have square shoulders / What is this metal ring in my box / Can I use my non tapered muzzle device? Why can't I get my muzzle device off? Can I use a Geissele Reaction Rod to remove it?

Photos from John Hollister on Facebook

Answer: Sig introduced a tapered muzzle shoulder to increase friction between the muzzle device and the barrel and thus improve the lockup of the muzzle device. Read more about it here. Sig also includes a ring shaped taper cap with each MCX in case you want to use a non-tapered muzzle device that requires timing against a square shoulder. If your muzzle device does not require timing, you can simply screw it all the way down and be good as long as it bottoms out on the muzzle itself. Rockset would be a good idea in that case. Additional taper caps can be ordered from Sig, and some other brands also make taper adapters that some people prefer them. Griffin Armament is one. Next Level Armament is another.

One side effect of the improved engagement of the tapered muzzle shoulder are that muzzle devices can seem to be stuck on, when they are in fact simply mounted as intended. If your muzzle device seems to be stuck, you may need to heat it up with a torch which will cause the metal to expand. Alternately you can soak the muzzle device in Kroil to loosen it. Under no circumstances should you attempt to change muzzle devices while the barrel is installed in the upper unless the barrel extension is taking the force of the torque. Unfortunately, only legacy barrel extensions are compatible with the Geissele Reaction Rod and similar devices due to the change to tapered bolt lugs.

Question: How do I change the dust cover on my Sig MCX series weapon system?

Answer: You must remove the forward assist/case deflector assembly to replace the dust cover. See below for details.

Question: How do I remove my forward assist assembly? Can I swap it for the Rattler case deflector?

Answer: Yes, you can swap the forward assist for the Rattler case deflector, also known as the forward assist delete. There are 2 screws on the underside of the brass deflector that hold it in place. However, be careful both when removing and installing them as they seem to be made of a softer metal and strip easily.

Thanks to eodsix and ayyyota

Deflector vs forward assist weight comparison and parts list by Fletch on Sigtalk.

FA - 1.45 oz

Low Profile .55 oz

2401436-01-R.....Deflector, Low Profile, Blk
2400420-01-R.....Insert, Deflector, Blk
2400730-R..........Pin, Coiled Spring, 1/8x5/8 SS
2400438-R..........Screw, FH, 6-Lobe plus, 4-40 x 1/4

Deflector/forward assist assembly screws should receive a light coat of Purple Loctite (#222) and are torqued to 10 in-lbs.

To swap between the FA and low profile deflector, all you need is the deflector assembly. All screws, the pin, and insert can be swapped from the old assembly if you so desire.

Question: What is the difference the LE (Law Enforcement "W" SKUs) and regular Virtuses?

Answer: Posted by user GLD1980
Also note that different contract runs may have different specs. So consider this a "general rule of thumb" situation.
Attached File


Question: What are the gas port sizes?

Answer: Screencap from TFB video posted by coltc
Attached File

Question: What are the generations of MCX?

Answer: Many!
Excluding prototypes. But for your reference, the Sig HALO PDW from 2013 https://soldiersystems.net/2013/05/16/sofic-sig-sauer-halo/ also an interesting post here about the 7.5" HALO with 10" handguard (text only no photos).

More early historical MCX photos
https://defensereview.com/sig-sauer-mcx-mission-configurable-weapon-system-dual-pistonop-rod-5-56x45mm-nato-and-7-62x39mm-russian-ar-15-with-integral-suppressor-introduced-at-ndia-sofic-2013-photos/
https://www.thefirearmblog.com/blog/2013/05/16/sig-mcx-pdw/
https://www.thefirearmblog.com/blog/2013/10/31/sig-mcx-pdw-photos-2/

Gen-0 Military: LVAW. Longer than legacy receiver rail, handguard with direct-thread M-Lok (no slots) with recoil lugs between the screw holes in various lengths, and carbon fiber versions as well. Aluminum handguards were issued in the kit with 8" and 10.5" lengths, with the 10.5" being used most often. Carbon fiber handguards were made by Lancer. 6.75” barrel, assumed auto adjusting gas block, the most common suppressor was the SRD762TI-10 which was an SRD338 suppressor with a bore optimized for .300Blk. Other suppressors also used (typically three in the kit, two titanium and one stainless steel. SRD762, SRD762TI, SRD762TI-10). Stocks could be anything from Kate Moss to telescoping to buffer tube. Issued as a complete kit so soldiers could customize as needed. AR-15 compatible bolt. No firing pin safety spring or latch. Worked with any AR-15 FCG. The direct thread M-Lok with recoil lugs was also seen on the Gen-1 MPX as known commercially as "add a rail". Generally used by CAG AKA Delta Force in any mission where the MP5 would have been used previously.



Clone video that talks about the differences and between the LVAW and legacy MCX as well as reviews the LVAW replica handguard.


Another clone video showing the "blockhead" clone stock.


Gen-0 Civilian: Cabela’s exclusive launch version. MCX legacy with original auto-adjusting gas block. AR-15 compatible bolt. No firing pin safety spring or latch. Pencil barrel. All barrels shared a common pistol length gas block location. Worked with any AR-15 FCG. Keymod handguard. As far as I can find, this was the only time a regular civilian pistol was offered with a 9" 5.56mm barrel. It was later an LE catalog item. Unknown numbers.


Gen-1 Civilian: Same as Gen-0 but with a two-position gas block.

Gen-2 Civilian: Same as Gen-1, but post recall: replaced AR-15 compatible bolt with HK416 style bolt (not compatible but looks similar). Also used HK416 style firing pin safety spring and firing pin safety latch. Mixed trigger compatibility. Milspec triggers work with factory safety latch. A Geiselle “G” latch was developed to work with Geiselle and similar hammer profiles. A so-called transitional handguard was briefly available on complete guns sold in various outlets and as standalone hanguards from CDNN. The transitional handguard was Virtus-like but had keymod slots and fit legacy receivers while introducing a slot to access the Virtus gas block.

Gen-3 AKA Virtus: Shorter and stronger receiver rail with more rugged handguard for better laser zero retention. Not compatible with previous handguards. Thicker barrel profile. Barrel extension and bolt head now use a tapered lug shape. Not compatible with previous barrels unless you also swap the bolt with it. 5.56mm barrels now have a more mid-length style gas block location. The added weight of the receiver, handguard, and barrel with gas block further out resulted in an overall heavier platform with more forward balance. Handguards now use M-Lok instead of Keymod. Some Keymod handguards were released as military contract overruns.

Noctis: This is a Bass Pro / Cabela's exclusive Virtus. Anodized black instead of cerakoted FDE or gray. Similar look to LE guns. Mil-spec style standard single stage trigger with a skeletonized hammer.

Rattler: Even shorter receiver rail than Virtus due to the short length of the barrel. Thus, handguards are not compatible between the Rattler and any other version. Removes the forward assist and has a brass deflector cut to accept telescoping stocks. 5.5” .300blk barrel has a removable flash hider and can be suppressed, however it’s been reported as having high back pressure when using traditional baffle style suppressors and supersonic ammo. Works best with low back pressure suppressors or subsonic ammo. The flash hider on the 5.5” 5.56mm barrel should not be removed and is not intended to be shot suppressed. Trialed by NSW, but not fully adopted in part due to poor results from salt water corrosion testing. Military 5.56 barrels were intended primarily for simunition training. Eventually adopted by SOCOM with both barrels.


Canebrake: This is a Rattler with suppressor width handguards and a fake suppressor.

SUR300: Virtus upper with 9” .300blk barrel and permanently attached suppressor to bring the overall length over 16” to make a one-stamp gun. https://www.silencershop.com/sig-sur300-suppressed-upper.html

TACOPS: TALO distributor exclusive Virtus pistol with 6.75” .300blk barrel and suppressor width handguard and fake suppressor. https://lipseys.com/itemdetail?itemno=SIPMCX300B6BTAC

SURG (Military only at this time): Suppressed Upper Receiver Group. Military trials contract winner. 11.5” 5.56mm upper with M4 receiver adapter and minimalist plus stock. Sig parts shown in coyote 498 cerakoted as required by the contract. Suppressor had a safety shield to prevent burn injuries and equipment damage.  https://soldiersystems.net/2018/09/20/live-fire-ussocom-suppressed-upper-receiver-group-surg/

MCX Spear: Next Generation Squad Weapon – Rifle. Based on the MCX-MR prototype submitted for the Compact Semi-Automatic Sniper System program that was chambered in .308. The MCX Spear is chambered in a hybrid brass cased high pressure 6.8x51mm round with a steel base. Uses SR-25 magazines.


MCX Spear-LT: Next generation of the MCX standard size platform. Initially offered only in coyote anodized finish. Several changes from the previous Virtus generation. Lighter than Virtus, but not as light as Legacy. Handguard stabilized by a screw at the top rear. Bolt carrier now compatible with most AR-15 hammers. Lower now has ambi bolt catch/release. First version with generally available 7.62x39 configurations in 11.5 and 16". 7.62x39 has been in prototype form for at least a decade, and has seen limited distribution on certain Virtus guns in late 2022.

Made links hot, added reference to Sig HALO PDW.

Added more links to early photos.

@coregon have I missed anything that isn't classified?

Were there Keymod Virtus rails, or are you talking about the Legacy rails with Keymod that are more Virtus-y?

I mean, if there's keymod Virtus rails, then I have to go find one.

I forget which, but I think they were a contract requirement for a European contract.

However, Googling the crap out of it, I can't find where I read that. I will go ahead and remove that until re-confirmed.

I will add a part about the transitional handguard.

Added more links and another video.

Added trigger compatibility. How did I forget that one?

Added a bit about the muzzle taper.

Not sure what else to add at this point. I was thinking about adding something about the colors, but even I'm not clear on that.

Basically, legacy MCX was shipped black anodized, but demo units were shown tanodized. All upper conversion kits are black anodized. All LE/Mil guns are black anodized unless special ordered in whatever color the contract requires (the Berlin guns come to mind). Civilian Virtus guns are Cerakote Elite Coyote or some shade of gray. Civilian Rattler could be black or FDE. Canebrake could be grey or coyote like the Virtus. Or at least that's what I can remember at this point. Anybody feel free to jump in and correct me. Tacops are usually black from what I've seen. But I saw one grey that was claimed to be a Tacops.

Supposedly coyote Virtus was discontinued in 2019 but people still claim to be finding them in stores.

I saw a FDE Virtus 5.56 rifle with a 2021 build date on the package. They are still being manufactured but in extremely small quantities.

Can the Rattler use a normal AR15 aftermarket charging handle? There seems to be some debate about it and I was hoping to get it cleared up. I'm still waiting on my SBR Rattler so I can't try it out myself.

No. The MCX family uses proprietary charging handles.

Say what? You can use any aftermarket AR-15 charging handle with the MCX.

Does this look like a standard AR15 charging handle to you?
https://www.sigsauer.com/sig-mcx-charging-handle-assy-ambi-large-latches.html

Yes it does. And the MCX charging handles aren't proprietary.

https://geissele.com/airborne%20-charging-handle-ddc.html

No, you can't. They look similar, but they're incompatible.

They are 100% proprietary. They are thicker and lack the little area the gas key goes into.

Another note, most aftermarket MCX charging handles work in the Rattler, but because of the pressure and recoil, the handle becomes dislodged sometimes, which is why companies say it's incompatible with Rattlers. This is partially true because it only occurs in 300 BLK. This issue is NOT present on the 556 variant of the Rattler.

Also, the Taccom recoil cushion works on every version of the MCX and every caliber.

So I guess everyone is wrong that is using G handles on their Virtual?

https://www.sigtalk.com/threads/questions-on-charging-handles-for-mcx.387624/

No. They were still a MCX-specific charging handle.

That issue with the charging handles dislodging does come up from time to time. Might need to add that one.

Clearly says "MCX". G, Radian, and others all produce/d MCX specific variant charging handles. Doesn't mean an AR15 charging handle will work in an MCX.

Do you even own a MCX? I’m guessing you don’t because  If you owned one it should be extremely obvious that a AR CH will not work. Which makes it even more that you would be so certain about about it based on your misunderstanding of some dudes question on another message board.

Yeah, he tried to be smart but fell flat. Can we have someone just prune all that nonsense out?

This is incorrect. There is no gas tube on the MCX.

Plus it won't even fit.

Added corrections to the firing pin latch safety and photos for the Sig muzzle taper. Also added Cabela's version video link.

Topics left to cover:

1. Charging handles: Not compatible with AR ones, and in Rattlers some will pop out.
2. The color situation. Anodizing vs cerakote, etc...
3. How do you change the dust cover? ( I have no idea )
4. How to swap the FA to Rattle deflector and vice-versa.
5. How to get off a stuck taper muzzle device.

The answers to Questions 3 and 4 are the same answer.  There are 2 screws on the underside of the Brass Deflector that hold that guy in place. With those out the deflector can be removed AND the dust cover can come out. It took me a considerable bit of wiggling to get all out and vice versa.  Putting it back together while holding the spring tension on the port cover was a pain but you can do it.

Thanks for contributing to the project. That's good info.

This. Also: be EXTREMELY careful, can't stress this enough, when re-torquing those screws (I don't know what the proper values are), as they strip/de-head SUPER easily.

These are the screws in question that secure the brass deflector/forward assist.  I couldn't tell they were difficult to remove, hence they weren't all that tight to begin with I suppose.  When I replaced them, I snugged them up to what I thought was appropriate.  As a matter of advice though, I would advise caution just as stated above.  If in doubt, snug up with some blue Locktite. This is not a part that you're gonna remove more than maybe once or twice at the most.  I mean, why would you?

Sorry for not updating lately. Work has been crazy.

Just added a part about muzzle devices, dust covers, and forward assists.

Cleaned up post about forward assist delete and added part numbers.

Per the Sig Armorer Maintenance Manual the two screws receive a light coat of Purple Loctite (#222) and are torqued to 10 in-lbs.  Hope that helps

Does anyone have further info on how the gas valve adjuster on the Virtus barrels line up with the cutouts on the Legacy receivers?  Since Legacy barrels are no longer offered, I’ve been wondering if the shorter gas systems on the 9” .300 and 11.5” 5.56 still line up with the Legacy cutouts.

Not sure. All I can say is that there are transitional handguards which have the longer cutouts. Not sure about the reverse.

Perfect! That's the kind of official info that we need. Appreciate the help. I've added it to the list.

As long as you have a transitional handguard, Virtus barrel, and Legacy upper, you'll be fine. Short ones exist for the 9", they're a bit harder to find.

Added another picture and some more details around the original LVAW.

Sorry for not keeping this updated guys. Busy season at work and kids back to school. You know it goes.

Great thread.  I'm glad I finally saw it.  I'm waiting for the ATF to approve my SBR right now.

It's worth noting that the Noctis Virtus rifle from Cabelas does not come with the 2 stage match trigger. They have a mil-spec type single stage trigger with a skeletonized hammer.

Added note about Noctis trigger and added the blockhead clone stock video.

Added some Rattler content.

Is there a function check to see if your AR trigger works?  Or is attempted live fire the only way to know?

As most firearms with a firing pin, you can run a rod/dowel down the barrel so see if you are getting firing pin engagement.

Thank you sir.

I’ve used a pencil many times using this same technique on my pistols.

@coregon

I found a SRD338ti-qd locally.  How does this can compare, dimension wise, to a legit LVAW suppressor as far as length?

Also, If I’m understanding correctly, it comes with a m18 muzzle brake.  I’d have to source a separate 5/8x24 muzzle brake to use this on an MCX .300 barrel?

Thanks.

Very close to same OAL.  ll of the Ti SRD cans have the same outside diameter.  Obviously, larger diameter bore in the end cap.  

If you were to nest it in a handguard, the only aesthetic difference would be the smooth end cap of the QD cans vs the wrench flat "teeth" of the DT cans (the LVAW cans have wrench "posts", instead of the teeth on something like the SRD762Ti)

You would have to source a 5/8x25 muzzle device separately.

Do MCX Virtus LE versions all come with Chromelined barrels?  It doesn't say.  I have an 11.5 inch Virtus in Coyote tan cerakote and it has a chromelined barrel. I got it in 2019 I believe. I called sig about it and they confirmed it was chrome.