The FN 509 is a full-size polymer-framed, striker-fired pistol using a frame with enhanced grip textures which enables faster follow-up shots in all conditions. The grip has an enhanced grip texture which enables faster follow-up shots and interchangeable backstraps adapted to all hand sizes. The slide and grip feature highly effective non-slip surfaces; magazine catch, slide release and safety catch are fully ambidextrous. This autoloading pistol with a preloaded striker mechanism has been tested successfully with a large number of 9x19mm ammunition types including FN® EP and EPT cartridges. The FN® 509 also features a tactile and visible loaded chamber indicator and a 17-round capacity magazine allowing for visual instant ammo check. The FN 509 does not have a manual safety, but a non-manual safety works in conjunction with passive safeties for safe operation. The FN 509 has a height of 139mm, an overal length of 188m long with a 101.66mm barrel, and an approx. weight of 753 gr. with an empty magazine, and approx. 957 gr loaded. The FN 509 uses a cold-hammer-forged stainless steel barrel with polished chamber and feed ramp.

The FN 509 pistol uses the current standard handgun 9x19 mm ammunition. The 9mm is one of the best rounds ever developed. It creates a low recoil, which allows for quick follow-up shots. The 9mm has a very high chamber pressure, which demands a high quality weapon, but that energy is used to propel the 9mm to a very high speed. Its high speed makes the 9mm useful at longer ranges and is thus very suitable for sub machine guns. The stopping power of the 9mm round, especially FMJ rounds, is moderate. However, it is quite effective in piercing objects and if necessary one can quickly fire a second round. The 9mm is quite small and allows for a high magazine capacity. The FN® 509 also features a tactile and visible loaded chamber indicator and a 17-round capacity magazine allowing for visual instant ammo check.