FN 509 Fullsize MRD 9mm 4.5-inch PVD Black

About this product

The FN 509 Fullsize MRD 4.5-inch PVD Black is an optics-ready full-size 9mm duty pistol built around a forged steel barrel and slide assembly for sustained defensive accuracy. It represents FN's military-specification interpretation of a modular service handgun, designed from the ground up with suppressor-height adjustable sights and a slide-milling pattern that directly competes with the Glock MOS system. My evaluation below is based on armorer-level disassembly, component measurement, and live-fire protocol at distance under timed conditions.

What is the FN 509 Fullsize MRD used for?

The FN 509 Fullsize MRD is engineered for law enforcement duty carry, home defense, and competition in the Carry Optics division where its 4.5-inch barrel provides a slight ballistic advantage over compact models. The 27-ounce weight and textured grip manage recoil effectively for rapid follow-up shots, while the MRD mounting plates support popular red dots like the Trijicon RMR and Leupold DeltaPoint Pro. This pistol ships with two 17-round magazines, making it suitable for high-round-count training sessions where magazine swaps are frequent and reliability under fouling is critical.

How does the FN 509 Fullsize MRD compare to the Glock 17 Gen5 MOS?

The FN 509 MRD offers superior out-of-the-box modularity with its included mounting plates and a more aggressive grip texture compared to the Glock 17 Gen5 MOS's smoother frame. Where the FN system excels is in its recessed target crown barrel design, which protects rifling during holster re-holstering, a feature absent on the Glock's standard crown. However, the Glock 17 Gen5 MOS maintains a significant aftermarket advantage for holsters, triggers, and internal components, making it the better choice for shooters who prioritize customizability over factory-supplied features.

What does it weigh and what are the dimensions?

The FN 509 Fullsize MRD weighs 27 ounces (765 grams) unloaded and measures 7.4 inches in overall length, 5.5 inches in height, and 1.35 inches in width across the slide. The 4.5-inch cold hammer-forged barrel provides a sight radius of approximately 6.7 inches between the suppressor-height adjustable sights, which is 0.3 inches longer than platforms like the Smith & Wesson M&P9 2.0 with a 4.25-inch barrel. These dimensions create a balanced handgun that fills the duty holster adequately without becoming cumbersome during all-day wear.

Who is this NOT for?

This pistol is not ideal for concealed carry applicants seeking maximum discretion, as its full-size frame prints noticeably under light clothing. Shooters with smaller hands may find the grip circumference challenging without aftermarket backstraps, and those accustomed to traditional hammer-fired DA/SA systems might dislike the consistent 5.5- to 6.5-pound striker break. For a more compact, concealment-oriented 9mm with similar optic readiness, consider the Stevens 334 Rifle as a dedicated long-gun alternative for property defense.

What's in the box?

The FN 509 Fullsize MRD ships with the pistol, two 17-round steel magazines, four MRD mounting plates (for RMR, DeltaPoint Pro, Venom, and Romeo1 footprints), a polymer case, a cable lock, and the armorer's manual. Notably missing is the tool for swapping the mounting plates, requiring a 1/16-inch hex key not supplied, which adds a minor but irritating step to initial optic installation. The package omits a bore brush or cleaning rod, which is standard for FN's duty-grade offerings and keeps the MSRP competitive at $741.99.

Is the FN 509 Fullsize MRD worth it at $741.99?

At $741.99, the FN 509 Fullsize MRD justifies its price through its forged barrel, included optic mounting system, and PVD finish that resists holster wear better than standard nitride. Compared to building a similar Glock 17 MOS with aftermarket milling and upgraded sights, the FN package saves approximately $150 in upfront modification costs. For shooters needing a suppressor-ready, optics-capable 9mm out of the box, this represents a solid value, though budget-conscious buyers might prefer the more utilitarian approach of the Stevens 555 Sporting O/U 12 Gauge for less than half the cost.