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Work order — FN PS90

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: high.

Shop difficulty: Tier 3 of 3

Mostly polymer, with a field strip into four groups that takes seconds. Past that, the hammer group and frame/trigger group are polymer assemblies held by pins and springs, and FN restricts them to its service centre or an armorer. The hammer group has to be in a particular state when it comes apart. With no published parts list and armorer-only internals, it sits with the hardest rifles here on uncertainty as much as evidence. Pew-Takedown's rating, not the manufacturer's.

Bench time: strip ~50 min, rebuild ~80 min (estimate). Actual: strip ______ rebuild ______

Special tools:

• Pin punches (polymer-safe) — Hammer and trigger group pins
• Sight adjustment tool — Rail sight

Watch for:

• FN limits the owner to the four-group field strip; anything further (hammer group, trigger group, barrel support) is service-centre/armorer work. (both; FN America, LLC)
• The hammer must be cocked (raised) before the hammer group comes out; if the trigger has been pulled with the moving parts out, the carbine has to be reassembled and re-cocked first. (both; FN America, LLC)
• The hammer group is a polymer assembly held together by pins and springs; owners advise leaving it to a gunsmith who knows the PS90. (both; Canadian Gun Nutz)

1. Clear and make safe

1. Point the carbine in a safe direction with your finger off the trigger and set the fire selector to 'S' (safe).
2. Pull the magazine catch rearward to release the magazine, then remove it.
   Warning: With the magazine on the carbine, the last two cartridges and the one in the chamber cannot be seen.
3. Pull the cocking handle completely rearward and release it to extract and eject any cartridge from the chamber. Repeat at least three times.
4. Pull the cocking handle completely rearward and hold it, then look through the magazine opening into the chamber to make sure it is empty.
   Tip: If a cartridge is still there, release the handle and pull it fully rearward again to extract it.
5. Release the cocking handle, make sure the magazine itself is empty, and remove all ammunition from the work area.

2. Disassemble

Field Strip

1. Complete the Clear and Make Safe procedure, with the magazine removed.
2. Pull the cocking handle completely rearward and release it, so the hammer is cocked.
   Warning: From here on, do not pull the trigger. The moving parts group and the hammer group can only be removed with the hammer cocked.
3. While pushing down on the barrel support lock, move the barrel support and upper rail group slightly forward, then slide it completely out of the frame.
4. Push the moving parts group forward through the magazine opening, then carefully slide it completely out of the frame.
5. At the rear, slide the butt plate up and off the frame.
6. While raising the hammer group release lever, pull the hammer group slightly rearward to release it, then slide it completely out of the frame.
   Tip: If the hammer group will not come out, the hammer has been decocked. Reassemble, cock the carbine with the cocking handle and start again.

Disassemble the Magazine

1. Remove the magazine and unload it completely.
2. With your index finger, press the magazine bottom lock through the hole in the magazine bottom plate.
3. Holding the bottom lock in, move the magazine bottom plate sideways until it disengages from the magazine body.
   Warning: The magazine bottom lock is under the force of the magazine follower spring.
4. Carefully release the pressure on the magazine follower assembly and take it out of the magazine body.
5. Hold the magazine body slightly inclined to remove the two rollers.

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.

No manufacturer statement of materials or finish is recorded. Identify each part's material before choosing prep and cure.

4. Reassemble

Disassemble the Magazine

1. Reverse the steps of "Disassemble the Magazine".

Reassemble the Magazine

1. Make sure all magazine parts are clean.
2. Insert the two rollers into the magazine body, one behind the other.
3. Insert the magazine follower assembly, oriented as shown on the pictogram on the side of the magazine body.
4. Push the magazine bottom lock in and hold it flush with the edge of the magazine body, then slide the magazine bottom plate back into position.
   Tip: The bottom plate only slides back on while the bottom lock is held flush.
5. Make sure the magazine bottom plate locks into position.

Reassembly

1. Make sure every group is clean.
2. Holding the hammer group release lever raised, slide the hammer group into the frame until it locks.
3. Slide the butt plate back down onto the rear of the frame.
4. Slide the moving parts group into the frame with its raised surface on top, until it locks into position.
5. Pressing the barrel support lock, slide the barrel support and upper rail group into the frame until it locks.
   Warning: Check that each group has locked into position before going on.
6. Fit an empty magazine, point the carbine in a safe direction, and pull the trigger to decock the hammer.
   Warning: Make sure the magazine and the chamber are really empty before you pull the trigger.

5. Torque

No manufacturer torque values are recorded for this firearm.

Notes: FN's owner's manual gives no torque values.

Fit or check on rebuild

• Hammer group dry — FN: hammer group components are never lubricated
• Function check — Safety, internal safety sear and trigger reset after the hammer group goes back
• Zero — Optic/sight on the barrel support must be re-zeroed

6. Function check

• Every group locks into position on reassembly; none can be pulled out without its lock or release lever.
• With an empty magazine fitted and the chamber empty, the hammer falls when the trigger is pulled.
• If the carbine does not fire or function, check that the fire selector is not on 'S', that it has been cocked, and that it has been reassembled correctly — FN's own troubleshooting order.

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

Sources

1. FN PS90 Autoloading Carbine Owner's Manual (1605, OM 3810054200) — FN America, LLC primary
   https://fnamerica.wpenginepowered.com/wp-content/uploads/2016/10/1605_PS90_Owners_Manual_v3.pdf
2. FN PS90 review/thread — Canadian Gun Nutz anecdotal
   https://www.canadiangunnutz.com/forum/goto/post?id=3776538

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.