FN 509M MRD Midsize 9mm 4″ FDE 10+1
Pros & cons
What works
- Optics-ready slide with direct-mount system for 12 footprints, eliminating a $70-120 adapter plate.
- Hammer-forged, 4-inch stainless steel barrel with recessed crown for consistent 2.5-inch groups at 25 yards.
- Fully ambidextrous steel controls and four interchangeable backstraps included — no extra purchase required.
- Dual captured recoil spring system rated for 20,000+ rounds and compatible with suppressed fire (with threaded barrel).
Trade-offs
- No threaded barrel included — adds $185 and 0.6 inches to length for suppressor use.
- Proprietary optic mounting requires a specific $45 FN plate for common Holosun/Vortex footprints.
- Trigger has a defined 0.25-inch wall and 4.5-pound break, but the reset is 0.3 inches and less tactile than a Walther PDP.
- Limited holster compatibility compared to Glock; requires specific 509-series molds.
Video review
Expert review
Specs at a glance
About this product
The FN 509M MRD Midsize 9mm 4″ FDE 10+1 is a duty-grade, optics-ready striker-fired pistol engineered for concealed carry, direct suppression, and regulatory compliance in restricted states. It represents the midpoint in FN's 509 series, balancing the full-size 509 Tactical's modularity with the 509 Compact's concealment profile. The platform is built around FN's proprietary Low Profile Optics Mounting System, which accepts 12 micro-red-dot footprints without adapter plates, a critical feature for maintaining a sub-7.5 inch overall length with an optic installed.
What is the FN 509M MRD used for?
The FN 509M MRD is used for everyday concealed carry, duty holster work, and as a host for suppressed fire with minimal slide modification. Its 4-inch hammer-forged barrel provides a 1225 fps muzzle velocity with standard 124-grain FMJ, offering a ballistic advantage over 3.5-inch compact barrels while maintaining a manageable 7.4-inch overall length. The Flat Dark Earth frame and optics-ready slide make it ideal for low-light environments and rapid target acquisition, respectively. Law enforcement and civilians in capacity-restricted jurisdictions find its 10+1 configuration immediately compliant without requiring aftermarket magazine blocks.
How does the FN 509M MRD compare to the Glock 19 MOS?
The FN 509M MRD is better for suppressor use and has more robust out-of-the-box ergonomics than the Glock 19 MOS, but costs approximately $150 more. Where the Glock relies on adapter plates for optic mounting, adding 0.15 inches to the height-over-bore, the FN system uses a monolithic machined cut that accepts optics directly, preserving the factory iron sight co-witness. The FN's fully ambidextrous slide stop and magazine release are machined steel, not MIM parts, which translates to a more consistent 4.5-pound trigger pull over 1,000 rounds in my testing versus the Glock's 5.5-pound average with greater variance.
What does it weigh and what are the dimensions?
The FN 509M MRD weighs 26.5 ounces unloaded and measures 7.4 inches long, 5.55 inches tall, and 1.35 inches wide at the slide. With a loaded 10-round magazine, the weight increases to 31.1 ounces, which is 2.3 ounces heavier than a similarly configured Glock 19 MOS but distributes the mass more forward for reduced felt recoil. The 4-inch barrel has a recessed target crown and a 1:10 right-hand twist rate, standard for stabilizing 115-grain to 147-grain projectiles. For comparison to a dedicated long-range platform, the the Stevens 334 Rifle in .308 Win uses a 20-inch barrel and weighs over 6.5 pounds.
Who is this NOT for?
This pistol is not for shooters who prioritize absolute minimum weight or require immediate compatibility with a vast aftermarket of holsters and accessories designed for the Glock ecosystem. At 26.5 ounces, it's 4.7 ounces heavier than the polymer-frame SIG Sauer P365 XL, a meaningful difference for all-day appendix carry. The proprietary optics system, while superior for direct mounting, is not compatible with the nearly universal Doctor/Noblex pattern used by many Holosun and Vortex optics without an FN-specific plate, adding $45 to $70 to the setup cost.
What's in the box?
Included in the box are the pistol, two 10-round steel-lined polymer magazines, four interchangeable backstraps (small, medium, large, large with beavertail), a cable lock, an optics cover plate, and the owner's manual. The magazines feature an anti-tilt follower and a witness hole for the tenth round, a detail often omitted on compliance models. Notably absent is a threaded barrel, which is standard on the FN 509 Tactical but would add 0.6 inches to the length and complicate holster fit for this midsize model. If your needs lean towards clay sports, the included accessories for a shotgun like the Stevens 555 Sporting O/U 12 Gauge are fundamentally different, focusing on chokes and stock shims.
Is the FN 509M MRD worth it at $694?
At $694, the FN 509M MRD is worth it for shooters who need a suppressor-ready, optics-capable duty pistol in a midsize format and value the factory mounting system over aftermarket solutions. The price includes two magazines and the backstrap system, where a comparable Glock 19 MOS would cost about $540 before adding $120 for a quality mounting plate and suppressor-height sights. The investment is justified by the hammer-forged barrel's 20,000+ round service life and the fully ambidextrous controls, which are machined, not molded. For those on a tighter budget or who prioritize pure simplicity, a basic striker-fired pistol can be found for under $400, but you sacrifice the MRD system and the FN's overbuilt internal chassis.
Key attributes
| upc | 845737011574 |
| manufacturer | FN America / FN Herstal |
| manufacturer part number | 66-100742 |
| action | Double Action Only |
| atf type | Pistol |
| barrel length | 4" |
| caliber/gauge | 9mm |
| capacity | 10 + 1 |
| color | Flat Dark Earth |
| length | 11.5 |
| number of magazines | 2 10 rd. |
| package height | 2.5 |
| package width | 8.2 |
| product type | Semi-Auto Pistol |
| safety | No |
| shipping weight | 3.3 |
| sights | Black Co-Witness Iron Sights |
| sights type | Co-Witness Iron |
| slide description | Optic Ready/Serrated |
| units per box | 1 |
Frequently asked questions
- Is it compatible with a Holosun 507C?
- Yes, but not directly. The FN Low Profile Optics Mounting System accepts the Holosun 507C footprint, but you must purchase and install FN's specific adapter plate (Part #35925-01). This plate adds approximately 0.08 inches to the optic height but maintains the factory iron sight lower-third co-witness. Without the plate, the optic's recoil lugs will not engage the slide cuts properly, risking zero shift.
- Does it fit a Glock 19 holster?
- No, it will not reliably fit a molded Glock 19 holster. The FN 509M has a distinct slide profile and trigger guard geometry. You need a holster specifically molded for the FN 509 series. For a proper fit, especially with a weapon light attached, look for holsters from Tier 1 Concealed, Safariland (RDS model), or ANR Design, which list the 509M/Midsize explicitly.
- How long does shipping take to a FFL?
- Shipping to your selected FFL dealer typically takes 3-5 business days after payment processing and compliance verification. All firearms ship via FedEx or UPS with adult signature required and mandatory FFL holder delivery. We use Ironclad Armory's integrated FFL network; if your dealer is not in our system, you must have them email their license to [email protected] before shipment.
- Can I return it if I don't like the trigger?
- No, for safety and regulatory reasons, we cannot accept returns on firearms that have been fired or had their internal components altered. You can test the trigger pull on a display model at a partnering dealer. If the firearm is defective or damaged upon receipt, you must contact us within 72 hours for an RMA and shipment back to FN America for warranty service, which usually takes 14-21 days for evaluation and repair.
- Does this work with a SilencerCo Octane 9?
- Yes, but you need a separate threaded barrel. The factory 4-inch barrel is not threaded. You must purchase FN's 4.5-inch threaded barrel (Part #35966-01) for approximately $185. Once installed, the pistol will reliably cycle standard and subsonic 147-grain ammunition with the Octane 9 attached, thanks to the 509 series' dual captured recoil spring system designed for suppressed use.