The FNA-B 43 submachine gun was designed and manufactured at the Fabbrica Nazionale d'Armie (National Arms Factory) in Brescia, with several thousands of guns made in 1943 and 1944. Despite its rather modern and compact appearance, it was built to pre-war standards of manufacturing and design. It had a delayed blowback action and a complicated trigger mechanism, as the gun fired from the closed bolt. Most of its parts were machined from solid steel, making it quite expensive to manufacture. Not surprisingly, its production was very limited in comparison to other contemporary submachine guns.
The FNA-B 43 submachine gun is delayed (retarded) blowback operated, selectively-fired weapon which fires from closed bolt. The delayed blowback action is achieved by using a Kiraly-type two-piece bolt with retarding lever, similar to that used in Hungarian 39M submachine gun. Since this weapon fires from the closed bolt, its firing pin has a separate spring, located inside the bolt, and a necessary linkage which releases the firing pin when bolt is fully in battery and the trigger is pulled. Fire mode selector is located on the left side of the gun, above the pistol grip. A separate safety lever is also located on the left side of the gun, above the trigger. Feed is from double stack, double feed magazines of the Beretta M38 pattern. The magazine housing is pivotally attached to the receiver and can be pivoted forward, so magazine would rest below the barrel and parallel to it – an useful feature when carrying a weapon with long 40-round magazine. The barrel of the gun is protected by a full length perforated jacket, made of steel, with integral muzzle brake – compensator at the front. The metallic shoulder stock folds down and forward, and rests on the right side of the gun when not required. The iron sights are fixed, with rear V-notsh sight factory zeroed for 100 meters range.