The FN 509 is a striker-fired handgun with a stainless steel slide with ferritic nitrocarburizing finish and a polymer frame. Black and flat dark earth (FDE) colorings are available. Barrel length is 4.0 inches (100 mm). The slide has serrations on both front and back, for easy slide manipulation. There is a full-sized grip, housing 17-round magazines; 10-round magazines are available for states with a high-capacity magazine ban.
The backstrap of the grip accepts interchangeable inserts; one arched and one flat. Instead of a manual safety, the 509 includes an integrated trigger safety (the articulated trigger type like the FN FNS) as part of its four passive safety systems: a striker block, a drop safety, a trigger disconnect and a trigger safety lever.
Magazine releases and slide locks are present on both side of the gun.