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Work order — FN 509
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.
| Work order # | |
|---|---|
| Customer | |
| Model on the bench | |
| Serial number | |
| Finish / colour | |
| Received | |
| Technician |
- FN Herstal limits owner disassembly: Field strip only: slide, recoil spring assembly and barrel off the frame (striker released); manual also covers magazine and backstrap changes.
- This work order is written for the FN 509. It is reported to cover FN 509 Tactical; FN 509 Compact; FN 509 LS Edge — if the firearm on the bench is one of those, check the torque notes: values do not always carry over.
Shop difficulty: Tier 2 of 3
59 line items in FN's standard-model list (some are day/night, backstrap and magazine variants); FN marks the unlock-block pin, fire-control pin, extractor pin and sights as replace-after-removal, restricts detail work to an armorer, and the manual-safety fire control carries a ball detent and spring. More parts and more traps than a Glock. 59 parts (FN 509 standard model, full-size frame, NMS and MS fire control; the parts list on this page (schematic pp. 58-59, list pp. 60-61); MRD/Tactical slide and Midsize/Compact lists omitted; 59 pieces). Pew-Takedown's rating, not the manufacturer's.
Bench time: strip ~25 min, rebuild ~35 min (estimate). Actual: strip ______ rebuild ______
Special tools:
- Replacement pins kit (unlock block retaining pin, fire control pin, extractor pin) — FN marks these replace-after-removal
- Frame fixture / support block (e.g. Apex FN disassembly fixture) — Driving the forward unlock-block pin without damage
Watch for:
- Several pins and the sights are single-use per FN: the unlock block retaining pin, fire control pin, extractor pin and front/rear sights carry 'replacement recommended after removal', so a shop needs new ones on hand. (reassembly; FN America, LLC)
- FN restricts disassembly past the field strip to a qualified armorer. (both; FN America, LLC)
- Manual-safety models carry a ball bearing and detent spring in the fire control, a captive detent that can fly. (both; FN America, LLC)
1. Clear and make safe
- Point the muzzle in a safe direction and keep your finger off the trigger.
- Press the magazine release and remove the magazine.
- Retract the slide fully to eject any chambered round and lock it open.
- Visually inspect the chamber and magazine well, then physically feel the chamber to confirm it is empty.
Warning: Look and feel. Both, every time. - Remove all live ammunition from the work area.
2. Disassemble
Field Strip
- Complete the Clear and Make Safe procedure and confirm the chamber is empty. Wear eye protection.
- Retract the slide and lock it in its rearward position using the slide stop.
- Remove the magazine and rotate the takedown lever downward.
- Release the slide stop and ease the slide forward.
- Keeping the rear of the slide and the rear of the frame in alignment, point the muzzle in a safe direction and press the trigger to release the striker. The slide moves forward slightly as it releases.
Warning: This pistol requires a trigger press to field strip — the striker has to be released before the slide will come off. Verify the chamber is empty, verify it again, and point the muzzle somewhere a round would not matter before your finger goes near the trigger.
Tip: FN's manual states the pistol must be completely unloaded to perform this step, and warns that failure to follow it could result in serious injury or death. - Pull the slide forward and away from the frame.
Tip: If the striker was inadvertently recocked, go back and release it again before trying. - Hold the slide upside down, grasp the small spring of the recoil spring assembly, and lift the assembly up and out of the slide.
- Remove the thread protector or muzzle device if the pistol has a threaded barrel.
- Grasp the barrel by its lug, lift it up and around the extractor, and slide it rearward out of the slide.
Tip: Up and around the extractor, then rearward. It does not lift straight out. - Stop here. FN states this level of disassembly is sufficient for a thorough cleaning.
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.
| Component separated | Out | Coated | Back in |
|---|---|---|---|
| Slide | |||
| Recoil spring assembly | |||
| Barrel |
| Component | Material | Factory finish | Source |
|---|---|---|---|
| Barrel FN 509 (standard, 4in), e.g. 66-100002 FN 509 NMS BLK/BLK | stainless steel | — | [5] |
| Frame FN 509 (standard, 4in), e.g. 66-100002 FN 509 NMS BLK/BLK | Polymer | — | [5] |
| Magazine FN 509 (standard, 4in), e.g. 66-100002 FN 509 NMS BLK/BLK | polymer | black | [5] |
The manufacturer's full parts list (59 parts) is on the reference page.
4. Reassemble
Reassembly
- Fit the barrel back into the slide, reversing the lift-and-around-the-extractor motion.
- Install the thread protector or muzzle device if the pistol has a threaded barrel.
- Place the large end of the recoil spring assembly against the recess in the front of the slide, compress it, and slide the back of the recoil spring guide onto the recess until it is centred on the front of the barrel lug.
- Guide the slide and barrel assembly onto the frame and pull the slide to its rearmost position, locking it with the slide stop.
- Rotate the takedown lever upward, locking the frame and slide together.
- Release the slide and let it travel forward, then test the slide stop, trigger, magazine release and manual safety if fitted.
5. Torque
| Fastener | Specification | Sequence | Source | Applied | Init. |
|---|---|---|---|---|---|
| Optic / slide-cap mounting screws (T10 Torx) through the MRD plate FN Optics Mounting System ("FN's patented Low Profile Optics Mounting System, U.S. Patent No. 10,352,654"), the MRD system the FN 509 owner's manual describes in section 11 with the same plates (501-5 |
22 in-lb 2.5 N·m · 1.8 ft-lb |
Remove lubrication from screws and holes; o-ring in slide recess; MRD plate over o-ring; optic or slide cap (with MRD insert if required); start both screws; tighten with finger pressure with the T10 wrench. Threadlocker: "DO NOT USE THREADLOCKER." (self-locking system) |
[3] PDF p. 1, step 9 (PDF p. 1) |
Reported by other sources — not the manufacturer
| Fastener | Reported value | Sequence | Source | Applied | Init. |
|---|---|---|---|---|---|
| Optic plate to slide screws (6-40 x 5/8 T10) FN 509 series (optics-ready) with Forward Controls Design OPF-FN509T, ACRO plate Reported by Forward Controls Design (secondary) — not the manufacturer |
13–15 in-lb 1.5–1.7 N·m · 1.1–1.2 ft-lb |
[2] | |||
| Aimpoint ACRO P1/P2 mounting screw FN 509 series with FCD OPF-FN509T, ACRO plate, Aimpoint ACRO P1/P2 Reported by Forward Controls Design (secondary) — not the manufacturer |
26 in-lb 2.9 N·m · 2.2 ft-lb |
[2] | |||
| Steiner MPS mounting screw FN 509 series with FCD OPF-FN509T, ACRO plate, Steiner MPS Reported by Forward Controls Design (secondary) — not the manufacturer |
20 in-lb 2.3 N·m · 1.7 ft-lb |
[2] | |||
| Red-dot optic mounting screws (6-32 flat head, supplied) into EGW rear-dovetail sight mount FN 509 (listed as fitting EGW's Sig #8-cut dovetail mount; EGW lists FN 509 Tactical as NOT fitting) with the EGW 'Trijicon RMR / SRO, Holosun 407c / 507c Mount for Sig Sauer P220, P226, P229, P320' ( Reported by Evolution Gun Works (EGW) (secondary) — not the manufacturer |
15 in-lb 1.7 N·m · 1.2 ft-lb |
Remove the rear sight, install the EGW mount in the dovetail, tighten the set screw if applicable, place the optic on the mount, then apply blue Loctite and torque the optic screws. Threadlocker: Blue Loctite on the provided mounting screws (EGW instructions) |
[6] | ||
| Red-dot optic mounting screws (supplied) into EGW rear-dovetail sight mount (Docter/Venom/Viper/FastFire footprint) FN 509 (listed as fitting EGW's Sig #8-cut dovetail mount; EGW lists FN 509 Tactical as NOT fitting) with the EGW 'SIG Sauer P220 P226 P229 P320 Red Dot Mount | Fits Vortex Venom, Viper & Burris FastF Reported by Evolution Gun Works (EGW) (secondary) — not the manufacturer |
15 in-lb 1.7 N·m · 1.2 ft-lb |
Remove the rear sight, install the EGW mount in the dovetail, tighten the set screw if applicable, place the optic on the mount, then apply blue Loctite and torque the optic screws. Threadlocker: Blue Loctite on the provided mounting screws (EGW instructions) |
[7] |
Notes: The manufacturer's documents give no torque value. They say: "Start both mounting screws and tighten them down clock-wise with finger pressure with the provided T10 Torx wrench." (FN 509 Owner's Manual (66-100498 R2, 2603), PDF p. 25 (section 11, optic installation step 9)) "Thread the suppressor or muzzle device on to the barrel and tighten it following the manufacturers recommendations." (FN 509 Owner's Manual (66-100498 R2, 2603), PDF p. 27)
Fit or check on rebuild
- Install new pins and sights — FN marks the pins and sights 'replacement recommended after removal'
- Sight position / re-zero — Drift or pressed sights moved for coating shift point of impact unless witness-marked and returned to the same spot
- Full function check (trigger reset, safeties, slide lock-back, drop of striker/hammer only on trigger) — Standard after any fire-control strip; confirms springs and sears went back correctly
6. Function check
- Verify the pistol is unloaded with no magazine inserted before each check.
- Rack the slide and press the trigger. The striker should release cleanly.
- Hold the trigger to the rear, rack the slide, then release slowly and confirm the reset.
- Press the trigger again to confirm the striker releases.
- Confirm the slide locks open on an empty magazine and the magazine drops free.
If any check fails, the firearm is not safe to fire. Stop and see a licensed gunsmith.
If it won't go back together
| Symptom | Cause | Fix |
|---|---|---|
| The slide will not pull forward off the frame. | The striker has not been released, or was inadvertently recocked. | With the chamber verified empty and the muzzle in a safe direction, keep the rear of the slide and frame aligned and press the trigger to release the striker, then pull the slide forward. |
| The barrel will not come out of the slide. | It is being lifted straight out. The 509 barrel clears the extractor first. | Grasp the barrel by its lug, lift it up and around the extractor, then slide it rearward out of the slide. |
Sources
- FN 509 Owner's Manual — FN America, LLC primary
https://fnamerica.wpenginepowered.com/wp-content/uploads/2026/03/2603_FN_509-OM_66-100498_R2.pdf - OPF-FN509T, ACRO (product page) — Forward Controls Design secondary
https://www.forwardcontrolsdesign.com/OPF-FN509T-ACRO_p_517.html - Quick Setup & Reference Guide: FN Optics Mounting System (2603) — FN America, LLC primary
https://fnamerica.com/wp-content/uploads/2026/03/2603_Quick_Setup_Reference_Guide_FN_OPTICS_MOUNTING_final.pdf - FN 509 Owner's Manual (66-100498 R2, 2603) — FN America, LLC primary
https://fnamerica.com/wp-content/uploads/2026/03/2603_FN_509-OM_66-100498_R2.pdf - FN 509® — product page (Specs / Primary Features) — FN America, LLC primary
https://fnamerica.com/products/pistols/fn-509/ - Trijicon RMR / SRO, Holosun 407c / 507c Mount for Sig Sauer P220, P226, P229, P320 (EGW product page, installation instructions section) — Evolution Gun Works (EGW) secondary
https://egwguns.com/trijicon-rmr-sro-holosun-407c-507c-mount-for-sig-sauer-p220-p226-p229-p320 - SIG Sauer P220 P226 P229 P320 Red Dot Mount | Fits Vortex Venom, Viper & Burris FastFire | EGW (EGW product page, installation instructions section) — Evolution Gun Works (EGW) secondary
https://egwguns.com/vortex-viper-venom-sig-sauer-p220-p226-p229-p320-sight-mount-fits-burris-fastfire-and-docter - FN Patents | FN® Firearms — FN America, LLC primary
https://fnamerica.com/fn-patents/
| Reassembled and function-checked by | Date |
|---|
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.