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Work order — SIG Sauer MCX

Taking the firearm apart for a job such as refinishing, and putting it back together to specification. Generated from the Pew-Takedown entry last updated 2026-10-29. Confidence: medium.

Shop difficulty: Tier 2 of 3

The upper is simpler than an AR, with no buffer tube and a captive dual recoil-spring op rod. The lower carries an AR-pattern fire-control group and its detents. SIG reserves anything past user level for armorers, so the full strip has the same traps as the AR, which makes it Tier 2. 28 parts (ASSEMBLY-LEVEL only: MCX VIRTUS upper and lower receiver assembly parts lists (sub-assemblies, not small parts)). Pew-Takedown's rating, not the manufacturer's.

Bench time: strip ~40 min, rebuild ~60 min (estimate). Actual: strip ______ rebuild ______

Special tools:

• roll pin punches — Lower parts
• pivot pin detent tool — Pivot pin detent
• SIG armorer tools — Anything past user level (see MCX Technical Armorer's Manual)

Watch for:

• The bolt carrier and op-rod springs are under load and fly forward if the bolt catch is touched while the group is out. (both; SIG SAUER, Inc.)
• SIG gives no owner procedure beyond the bolt and extractor. Lower and fire-control detail strip is armorer-level (SIG MCX Technical Armorer's Manual). (both; SIG SAUER, Inc.)
• The AR-pattern hammer spring in the lower can be fitted backwards and fail to detent the trigger pin. (reassembly; Gun Tests)

1. Clear and make safe

1. Point the muzzle in a safe direction and keep your finger off the trigger.
2. Place the selector on safe.
3. Press the magazine release and remove the magazine.
4. Pull the charging handle fully to the rear and lock the bolt back if the rifle has a hold-open.
5. Inspect the chamber and magazine well with a light, then physically confirm the chamber is empty.
   Warning: Look and feel. Both, every time.
6. Remove all live ammunition from the work area.

2. Disassemble

Field Strip

1. Complete the Clear and Make Safe procedure and release the bolt forward.
2. Push out the rear takedown pin and pivot the upper receiver open on the front pivot pin.
3. Push out the front pivot pin and separate the upper from the lower receiver.
4. Pull the charging handle rearward and withdraw the bolt carrier group and recoil spring assembly out of the upper.
   Warning: The dual recoil springs are under compression.
5. Remove the firing pin retaining pin, tip out the firing pin, rotate and lift the cam pin, and withdraw the bolt.
   Warning: Keep the cam pin with the rifle.
6. Withdraw the gas piston from its cylinder, noting orientation.

3. Refinish

The components the procedures above separate. Coating, masking and cure are outside this database; follow your coating system's instructions. A part the manufacturer marks "factory fit" should not be swapped or reworked.

The manufacturer's full parts list (28 parts) is on the reference page.

4. Reassemble

Reassembly

1. Reinstall the gas piston in its correct orientation.
2. Fit the bolt into the carrier, install and rotate the cam pin, then the firing pin and its retaining pin.
3. Insert the bolt carrier group and recoil spring assembly into the upper receiver and seat them.
4. Mate the upper to the lower receiver and press both pins home.
5. Cycle the action several times and perform the function check.

5. Torque

Reported by other sources — not the manufacturer

Notes: The manufacturer's documents give no torque value. They say: "Reinstall the clamp and tighten the screw snug plus ¼ turn." (SIG MCX Owners Manual: Handling & Safety Instructions, PDF p. 54 (Collapsible Stock Installation/Removal)) "Tighten the screw until snug plus 1/4 turn." (SIG MCX Owners Manual: Handling & Safety Instructions, PDF p. 56 (3.8.5 Folding Stock Installation/Removal))

Fit or check on rebuild

• Hammer spring orientation — AR-pattern FCG.
• Function check (safety, trigger reset, feeding, extraction) — Standard after any detail strip. No single source.
• Re-zero sights or scope — Sights and optics come off for coating, and point of impact must be re-confirmed. This is a standard check, so no source.

6. Function check

• Verify the rifle is unloaded with no magazine inserted before each step.
• Cycle the action by hand and confirm the bolt locks and returns fully to battery.
• With the selector on safe, press the trigger. The hammer must not fall.
• Move the selector to fire, cycle the action, and press the trigger. The hammer should fall.
• Hold the trigger back, cycle the action, and release slowly to feel the disconnector reset.
• Confirm the magazine seats and locks positively and any hold-open engages on an empty magazine.
• Confirm the gas regulator is on the correct setting for how you are shooting the rifle.

If any check fails, the firearm is not safe to fire. Stop and see a licensed gunsmith.

Sources

1. SIG Sauer MCX Operator’s Manual — SIG Sauer, Inc. primary
2. Torque Specs (Torque Specs by Brand — Sig Sauer MCX) — Fix It Sticks (compiled reference list) tertiary
   https://store.fixitsticks.com/pages/torque-specs
3. SIG MCX Owners Manual: Handling & Safety Instructions — SIG SAUER, Inc. primary
   https://www.sigsauer.com/media/sigsauer/resources/MCX.pdf
4. SIG MCX VIRTUS Operator's Manual: Handling & Safety Instructions (2402117-01 REV 00) — SIG SAUER, Inc. primary
   https://www.sigsauer.com/pub/media/sigsauer/resources/OPERATORS-MANUAL-SIGMCX-VIRTUS-ENGLISH-2402117-01-REV-00-LR.pdf
5. SIG MCX-SPEAR LT Operator's Manual: Handling & Safety Instructions (2402988-01 REV00) — SIG SAUER, Inc. primary
   https://www.sigsauer.com/media/sigsauer/resources/OPERATORS-MANUAL-MCX-SPEAR-LT-2402988-01-REV00-WEB-FILE.pdf
6. Arisaka Zero Retention Clamp — MCX Virtus (SIG Standard / SIG SD Suppressor / MI Standard / MI Suppressor handguards), dated 07/25/2023 (ArisakaZRC-Virtus-03) — Arisaka Defense secondary
   https://arisakadefense.com/content/ArisakaZRC-Virtus-03.pdf
7. (Gunsmithing the AR-15 #2) The 30-Second AR Check — Gun Tests secondary
   https://www.gun-tests.com/?p=26007

Educational guide only. Some information comes from non-authoritative sources. Always consult your manufacturer's manual and a licensed gunsmith. Verify your firearm is unloaded before every procedure. Full disclaimer: https://github.com/Fundamental-build-LLC/Pew-Takedown

Legal notice: this material is third-party information compiled from sources of varying reliability and is provided for general informational and educational purposes only. It is not gunsmithing, technical, safety or legal advice, it is not verified or endorsed by any manufacturer, and it must not be taken as fact. Any work on a firearm beyond the routine field stripping described in its own owner's manual should be performed only by a licensed and insured gunsmith. You use this material entirely at your own risk. Fundamental Build LLC and the project's contributors make no warranty of any kind and accept no liability for death, injury, damage or loss arising from its use. Full terms: LEGAL.md.