The FNAB-43 is an Italian designed and developed submachinegun manufactured from 1943 to 1944. The first prototype was built in 1942 and the 7,000 built by the FNA-B (Fabbrica Nazionale d'Armi di Brescia, "Brescia National Arms Factory", hence the name) were issued to German and Italian RSI [ˌɛrreˌɛsseˈi] (Repubblica di Salò pronounced [reˈpubblika di saˈlɔ]) units fighting in Northern Italy. The FNAB-43 was an expensive weapon to manufacture as it used extensive milling and precision engineering in its manufacture.
The FNAB-43 uses a lever-delayed blowback system firing from the closed bolt position. The bolt is a two-piece unit with a pivoted lever interposed between bolt head and body. On firing the bolt head retracts and begins to rotate the lever, the base of which is against a lug in the body. This lever is pivoted to delay the opening movement to allow the bullet to leave the barrel. The breech pressure then drops before the lever has completed its rotation. The movement of the lever then presses the free end against the bolt body and accelerates the bolt's movement to the rear. The base of the lever then pulls clear of the lug and the whole bolt unit continues to recoil as one piece. On returning, the lever again engages the lug and pivots forward and in doing so removes a coupling which allows the firing pin to move only when the bolt is fully forward. This unusual and complicated system allows the rate of fire to be kept down to very practical 400 rpms without having to use a heavy bolt or strong spring.
The FNAB-43 also uses a muzzle brake and compensator built into the barrel casing similar to some Russian weapons, and the magazine housing that is hinged so that the magazine can lie beneath the barrel similar to the French MAT-49. The single metal bar stock folds upwards allowing the weapon can be compactly carried.