The FN 509 Midsize MRD is a striker-fired 9mm pistol with a 4-inch barrel and 15+1 round capacity. It weighs 26.5 oz (1.6 lbs) unloaded and measures 7.4 inches overall. The FDE finish and flat dark earth polymer frame give it a distinctive look. The MRD designation means the slide is milled for direct optic mounting using FN's Low Profile Optics Mounting System, which supports adapters for over ten optic brands without needing additional modification. 
 

Key Specifications 

• Brand: FN America 
• Model: FN 509 Midsize MRD 
• Caliber: 9MM Luger (9X19 PARA) 
• Action: Striker-fired 
• Capacity: 15+1 rounds 
• Barrel Length: 4 inches 
• Overall Length: 7.4 inches 
• Weight: 26.5 oz (1.6 lbs) 
• Sights: All-black co-witness iron sights 
• Optic Mounting System: FN Low Profile Optics Mounting System with adapter plate 
• Finish Color: Flat Dark Earth (FDE) 
   

Detailed Features 

 
The striker-fired mechanism delivers a consistent trigger pull shot to shot. The cold hammer-forged stainless steel barrel is more durable than button-rifled alternatives and holds its accuracy over high round counts. The deeply recessed target crown protects the muzzle from contact damage that can degrade accuracy. The grip texture is aggressive enough to maintain control during fast firing sequences, and the ambidextrous controls work from either hand. The co-witness iron sights allow backup aiming when an optic is mounted.

The FN 509 Midsize MRD in FDE draws broad praise for build quality and reliability. Owners consistently cite the solid construction, comfortable grip, and smooth trigger as strengths. The optics-ready mounting system is a practical plus -- multiple red dot brands mount directly without adapter gymnastics, which competitive shooters and tactical users particularly appreciate. Accuracy at typical handgun distances gets strong reviews.

The midsize format generates some discussion: it sits between subcompact and full-size, which works well for shooters who want carry capability without giving up too much capacity, but it's not the optimal choice for dedicated pocket carry or full-size competition use. Weight is occasionally flagged as higher than some buyers expect for extended carry. Holster availability is more limited than for Glock or M&P-pattern pistols, which requires some hunting.

A subset of owners would prefer more aggressive grip texturing. Overall, the 509 Midsize MRD is well-regarded as a capable, reliable optics-ready 9mm that performs confidently across self-defense, competition, and range use.