The FN 509 Midsize MRD is a striker-fired 9mm pistol with a 4-inch cold hammer-forged stainless steel barrel and 15+1 capacity. It ships with two magazines. The MRD designation indicates the FN Low Profile Optics Mounting System, which uses plate adapters to accommodate over ten different optic footprints directly on the slide. 
 

Key Specifications 

• Brand: FN America 
• Model: 509 Midsize MRD 
• Caliber: 9mm Luger 
• Action: Semi-Auto Striker Fire 
• Capacity: 15+1 rounds (includes 2 magazines) 
• Barrel Length: 4 inches 
• Overall Length: 7.4 inches 
• Weight: 26.5 oz 
• Frame Material: Polymer 
• Sights: FN Low Profile Optics Mounting System with plate adapters 
• Grip Type: Standard with interchangeable backstraps 
• Finish Color: Black 
• Condition: New in Box 
   

Detailed Features 

Slide and magazine releases are fully ambidextrous, which is a practical feature FN carries across the 509 line. The grip has aggressive texture panels and accepts interchangeable backstraps for size adjustment. The barrel uses a polished chamber and feed ramp for reliable cycling. FN's extended magazines (17 rounds and higher) are compatible with the Midsize frame using corresponding magazine sleeves, which expands the capacity options without buying a separate pistol. 
 

Target Audience 

The 509 Midsize splits the difference between FN's full-size and compact 509 variants. The 4-inch barrel and mid-size grip fit shooters who want more than a subcompact for carry or home defense but don't need the bulk of a full-size duty pistol. The optics-ready platform appeals to shooters running red dots for competition or duty use. 
 

Usage Scenarios 

• Concealed carry where the mid-size frame balances concealability with capacity 
• Home defense with a red dot mounted via the MRD plate system 
• Competition in divisions that allow optics-ready pistols with standard capacity magazines

Owner feedback on the FN 509 Midsize MRD is consistently positive, with the optics system and reliability getting the most attention.

The MRD optic mounting system is the most-discussed feature. Owners appreciate that it uses low-profile plates that keep the sight line close to the bore axis rather than requiring a riser. The compatibility with over ten optic footprints means most popular micro red dots fit without an adapter problem.

Ambidextrous controls are noted as genuinely functional rather than cosmetic. Left-handed owners specifically mention that both the slide stop and magazine release operate smoothly from either side.

Reliability is rated highly across a range of 9mm loads. Most owners report clean function from the first range session, and long-term owners note that the cold hammer-forged barrel holds up well after high round counts.

The trigger gets generally positive marks. The pull is consistent and the reset is short enough for follow-up shots, which serves both carry and competition applications.

The main criticism is price. FN's pricing sits above several competing pistols with similar feature sets, and some owners feel the premium is harder to justify compared to alternatives. Holster and aftermarket accessory availability is also noted as more limited than Glock or SIG equivalents, though the 509 ecosystem has grown.

The interchangeable backstraps add practical value for households where multiple users shoot the same pistol.