The FN 509 Midsize is a DAO semi-automatic 9mm pistol with a 4-inch cold hammer-forged barrel, 15+1 capacity, and a polymer frame with Picatinny rail. The ambidextrous controls, interchangeable backstraps, and matte black stainless slide make it adaptable for personal defense and competition. 
 

Key Specifications 

• Brand: FN America 
• Model: 509 Midsize 
• Caliber: 9mm Luger 
• Action: Semi-Auto (DAO) 
• Capacity: 15+1 rounds (includes two 15-round magazines) 
• Barrel Length: 4 inches 
• Overall Length: 7.4 inches 
• Weight: 26.5 oz (1.5 lbs) 
• Sights: Fixed 3-Dot, contrast style 
• Frame Material: Polymer with Picatinny Rail 
• Slide Finish/Material: Matte Black Stainless Steel 
• Grips: Textured Black Interchangeable Backstraps 
   

Barrel Performance 

The cold hammer-forged stainless steel barrel has a recessed target crown that protects the muzzle and promotes consistent accuracy over time. 
 

Enhanced Ergonomics 

Interchangeable backstraps give shooters a customizable fit. The grip texture improves control during rapid strings of fire. 
 

Ambidextrous Controls 

Fully ambidextrous slide stop and magazine release buttons work equally well for left- and right-handed shooters. 
 

Practical Accessories 

Two magazines and a soft pistol case are included. 
 

Usage Scenarios 

1. Home Defense: 15+1 capacity in a compact frame handles defensive use without bulk. 
2. Sport Shooting: Well-balanced for competition applications where accuracy under time pressure matters. 
3. Concealed Carry: The 26.5 oz weight and 7.4-inch length are manageable for daily carry with appropriate gear. 
4. Professional Use: FN's track record in law enforcement and military contracts supports its reliability credentials.

The FN 509 Midsize Gen 1 is well-regarded in the compact pistol market, with a few common criticisms.

Build quality is the consistent highlight. FN's manufacturing reputation shows up in the 509's solid construction and the cold hammer-forged barrel, which owners frequently mention as a contributor to lasting accuracy. The midsize frame strikes a practical balance between carry-friendly dimensions and a grip that supports good control during range sessions.

The 15+1 capacity is seen as a genuine advantage for daily carry, giving more rounds in a package that still fits under most concealment setups. Most owners report the grip is comfortable across different hand sizes.

On the critical side, trigger feel is the most debated aspect -- some find the DAO pull acceptable, others want a lighter or crisper break and look at aftermarket options. Factory sights get occasional criticism as a weak point, with some users opting for aftermarket replacements.