The Model 43 was designed at Armaguerra in 1942 and put into production at FNA-B in 1943. It was issued to both RSI troops and German occupying forces. Due to the costs of manufacturing the weapon, only a limited number were ever produced; the exact number is disputed, but never exceeded 10,000 units.
The FNA-B Model 43 was a delayed blowback-operated submachine gun. The bolt was in two seperate parts; the bolt head and the main body, operated by a lever. Upon firing, the two parts would come together and release a lock on the firing pin, allowing it to strike the chambered round. The blowback action would then send the bolt body backwards while the bolt head was held.
The Model 43 was made to a very high standard and every component was machined. This ensured it was sturdy and well-built but also drove up the production time considerably. The barrel featured a perforated jacket and muzzle compensator, and the magazine housing was on a hinge to allow the magazine to fold up underneath the barrel. The stock was a single strut that swiveled on a rotating axis.