Like the company's full-size FN 509, the 509 Midsize is equipped with a 4-inch barrel and features the same slide length, sight radius and Picatinny frame rail as the initial 509 offering. However, the frame height on the new Midsize is shortened, reducing the overall height of the gun from 5.56 inches to 5.2 inches. Additionally, flush-fit magazine capacity is reduced from 17 rounds to 15 rounds.
The new release of the Midsize model also allowed FN to address several small features on the gun. The sculpted contouring around the magazine-release button has been reshaped to allow for easier activation of the release. The trigger face is flatter, allowing for a more-controlled trigger press, and the slide-stop lever includes texturing to allow for better tactile engagement. In a boon for current owners of either the full-size 509 or the 509 Tactical, the Midsize is backwards-compatible with all higher-capacity 509 magazines, including the 17- and 24-round extended magazines that ship with the company's Tactical model.
Operating with the same reliable, striker-fired action tested up to 1 million rounds on the original, full-size 509, the FN 509 Midsize is equipped with ambidextrous controls, ledge-style, luminescent, three-dot sights, a loaded-chamber indicator and cocking serrations at the front and rear. The pistol is features a cold hammer-forged barrel made from stainless steel and finished with an accuracy-enhancing target crown, as well as a polished chamber and feed ramp. Each Midsize model ships with two 10- or 15-round magazines, depending on capacity restrictions in the buyer's home state. The suggested retail price on the gun is $649.