Thank you, sir. Or ma'am.

Does the MCX Spear LT upper absolutely require the use of the MCX Stock Adapter to use on an AR15 lower? I read a review that said you could plop the Spear LT upper onto an AR lower but it just would look weird since it sits up taller. I was wondering about how the MCX recoil rod would be supported or interfere with an AR buffer.

Yes. You need the adapter to support the recoil assembly or else you pull a 'NutnFancy.'

MCX: Generation 3/SPEAR LT

Thank you! The photo makes sense!

Has anyone found any torque values for the MCX stock adapter? In a few other threads, someone called SIG and they told them to snug everything up - no torque values. Someone else said their Virtus armorer's manual say 70 in-lb for the bumper screw (which seems excessive).

To me the buffer adapter has that wrench flat, and its design makes me think torquing to 40 ft-lb ala rifle buffer extension should work? The bumper screw, probably 40-45 in-lb?

I did find all of the other screw torque values (for the screws I could find).

Handguard Link Screw 2400439-R, #8-32, 10 in-lb (as per SIG as well)
Cam Path Insert Screw, 2400180-R, #6-32, supposedly 35 in-lb according to SIG, but that seems like way too much torque for a steel 6-32 screw let alone SS.
Forward Assist Screw, 2400438-R, #4-40, 6-8 in-lb  according to SIG, but I saw someone's armorer manual excerpt showing 10 in-lb w/ Loctite 222.

@SpyHawk

I saw this on the forum the other day: https://www.ar15.com/forums/armory/MCX-Generation-3-SPEAR-LT/45-534561/&page=39#i6011026

I also saw a while back that someone was asking about the MCX uppers being larger than standard AR lowers (can't find it on the search and don't remember which sub-forum it's in).

I took a few pictures of what Virtus and SPEAR-LT uppers look like on a standard AR lower. I didn't see this in your first post and SIG themselves haven't really talked about it.

The Virtus and SPEAR-LT uppers are significantly "wider" and protrude out over the sides of the lower receiver when using a standard AR. I have not run into any function issues using either upper on the AR lower, however, the forward assist housing screws are clearly visible when mounted on a standard AR lower.

MCX VIRTUS [LEFT] - S&W MP15 [RIGHT]


MCX VIRTUS UPPER on MP15 LOWER


MCX SPEAR LT UPPER on MP15 LOWER

Yes, that was me. Thank you! I've heard this referred to as the muffin top. It's actually not that bad - it only hangs over part of the lower.

My issue is that my lower receivers have always been "in-spec" with AR15 uppers. Meaning whether it is an Aero, LaRue, or early 2000s RRA - the lowers always fit nice and tight, no issues. But the MCX Spear LT is more finnicky as the part that faces the front takedown has limited clearance. AR15 uppers for the most part don't have anything covering the front takedown lugs of the receiver. So there's no clearance issue.

After much investigation, it seems the lowers have a little too much material at the front that interferes with the MCX Spear LT upper. I don't see a reason for them to make the uppers with this limited space. These are the scalloped areas near the 2 roll pins and feed ramp insert.

However, another problem I'm now encountering is the handguard link is too tight because once the handguard screws are tightened, the handguard link tilts a little making for a very tight fit of the pin. I need to use a tool to push it out otherwise I am not able to remove it like on an AR15 upper. I tried loosening the HG link screws and HG screws, slipping the takedown pin through (easily slides through) and then tightening all of the screws - but this results in the pin being really stuck.

Is this normal? Does SIG want to use the HG Link to essentially keep everything snug (especially the HG)? can I ditch the HG link if I use the Arisaka ZRC?

I have a virtus upper and recoil asssembly. I have a 9 inch spear LT 300bo barrel. Are the BCGs compatible or do I need a certain one? Thanks

They are fine.

The gas regulator for the Spear LT 11" barrel is a JM01
Low Side port: 0.064" (recessed/slotted)
High Side port: 0.059"

Also the SIG taper adapter will not completely cover the shiny taper exposed when you remove the factory Clutch-Lok flash hider on the MCX Spear LT.

A much easier way (in my eyes, since I have had those tiny forward assist screws strip on me more than once - and one Virtus receiver I have has “tamper-proof” screws in there) is to fully loosen the barrel clamp, remove the barrel, and then drive out the roll pin on the bolt-head side of the barrel clamp (the pin that retains the bolts). This will allow the barrel clamp to pop off and you can then easily slide the dust cover rod forward on the receiver like you would on a standard AR upper.

There is even a small slot in the bottom of the forward assist that you can get a small pick into to help push that rod forward to get it started. I just used this method tonight to remove one from an LT upper to switch it to a different cover.