It is chambered in 9×19mm Parabellum featuring double-action operation and a Picatinny rail located forward of the trigger guard.
[8] The FN 509 is a striker-fired handgun with a stainless steel slide with ferritic nitrocarburizing finish and a polymer frame.
The backstrap of the grip accepts interchangeable inserts; one arched and one flat.
[9] Magazine releases and slide locks are present on both side of the gun.
‡ For the Compact MRD variant, 15-round extended magazines also available, which increase height to 5.2 in (130 mm).