This replacement or spare magazine is compatible with your fn 509 midsize.
Caliber: 9mm Luger
Capacity: 15rd
Finish: Black
Model: OEM
Material: Stainless Steel
Type: Detachable
Model Fit: 509 Midsize
Brand Fit: FN
Capacity: 15
Caliber: 9mm Luger
Capacity: 15rd
Finish: Black
Model: OEM
Material: Stainless Steel
Type: Detachable
Model Fit: 509 Midsize
Brand Fit: FN
Capacity: 15