The FN 509 Midsize MRD is a striker-fired 9mm Luger semi-auto pistol with a 4-inch cold hammer forged barrel, 7.4-inch overall length, 26.5 oz weight, and 10+1 capacity. Two magazines are included. The optics-ready slide accepts more than ten optics via FN's low-profile mounting system with plate adapters — no gunsmithing required. 
 

Key Specifications 

• Brand: FN America 
• Model Name: 509 Midsize MRD 
• Caliber: 9mm Luger 
• Action: Semi-Auto Striker Fire 
• Barrel Length: 4 inches 
• Overall Length: 7.4 inches 
• Weight: 26.5 oz (1.6 lbs) 
• Capacity: 10+1 rounds 
• Magazines Included: 2 
• Grip Type: Black Interchangeable Backstrap 
• Sights: FN Low Profile Optics Mounting System with plate adapters 
   

Detailed Features 

The cold hammer forged barrel handles extended round counts without losing precision. Trigger pull weight runs 5.5 to 7.5 lbs with a consistent break. The textured interchangeable backstraps adjust fit across different hand sizes. The frame accepts standard 509 accessories.

The FN 509 Midsize MRD draws clear opinions on both sides.

Ergonomics are regularly praised — the grip texture gives reliable traction without being rough on bare hands, and extended sessions don't cause the fatigue some compact pistols produce. Accuracy across multiple distances comes up in most positive reviews, and the optics-ready capability is a major draw for shooters who run red dot sights.

Build quality gets consistent marks. Owners report the materials feel solid and compare favorably against other mid-priced striker-fired pistols.

On the critical side, some concealed carry users find the midsize frame too large for daily carry, though the same dimensions appeal to range shooters. Trigger opinions split — some describe it as smooth while others call it heavier or less crisp than expected. Price lands at the higher end of the category, and aftermarket accessories and holster options are more limited than for Glock or Smith & Wesson M&P.