The FN 509M is a full-size 9mm semi-automatic pistol with a Double Action Only trigger, 15+1 capacity, and a Flat Dark Earth finish. It's built around FN's 509 platform with cowitness iron sights and an interchangeable backstrap grip. 
 

Key Specifications 

• Brand: FN 
• Model Name: 509M 
• Caliber: 9mm Luger 
• Action: DAO (Double Action Only) 
• Capacity: 15+1 rounds 
• Barrel Length: 4 inches 
• Overall Length: 12 inches 
• Weight: Approximately 26.5 oz. 
• Sights: Cowitness iron sights 
• Grip: FDE Interchangeable Backstrap 
• Safety Features: None (DAO action) 
• Finish Color: Flat Dark Earth 
• Condition: New in Box 
   

Features 

The DAO trigger delivers a consistent pull on every shot with no manual safety to manage. Cowitness iron sights are designed to align with red dot optics, so you can run both simultaneously without losing your backup. The interchangeable backstrap system lets you adjust grip thickness to fit different hand sizes. The polymer frame keeps weight manageable at 26.5 oz. The FDE finish is corrosion resistant. 
 

Who It's For 

The FN 509M works well for home defense, sport shooting, and range training. The 15+1 capacity gives adequate firepower for defensive use. Competitive shooters will appreciate the smooth DAO trigger and optics-ready cowitness sights. Those who prefer striker-fired triggers should note this is a dedicated DAO design.

Owners of the FN 509M FDE generally respond well to the pistol, with a few recurring areas of debate.

The interchangeable backstraps get consistent praise -- shooters with different hand sizes appreciate being able to dial in the grip fit. Build quality is frequently cited as a strength: owners describe it as solid and durable, and reliability across different ammunition types is a common positive point. The 15+1 capacity is well-regarded for a full-size defensive pistol.

The FDE finish draws positive aesthetic comments. Many buyers chose it specifically to stand out from standard black pistols.

On the critical side: the DAO trigger is a dividing issue. Some shooters find it less crisp than striker-fired alternatives, which affects their accuracy. The pistol's weight is noted by some as heavier than what they prefer for carry.

Price generates debate -- some buyers consider it fair for an FN product, while others find it expensive compared to similar 9mm options from competing brands. Aftermarket support for accessories and upgrades is more limited compared to the most popular 9mm platforms, which is worth factoring in for anyone who likes customizing.