With development stemming from 1940, this Portuguese submachine gun evolved in the post-World War 2 years into the "FBP" (Fabrica de Braco de Prata). The war introduced all sorts of capable designs, functions, and components to which several were simply incorporated into the original Portuguese design. Design of the weapon was attributed to Portuguese Army Major Goncalves Cardoso and the weapon led a healthy frontline service life nearing 30 years.
The FBP utilized the telescoping bolt, barrel, and general construction (steel pressings) of the German wartime MP40 submachine gun along with the pistol grip, trigger mechanism, and wire stock of the famous American M3 "Grease Gun". The resulting FBP was chambered for the ubiquitous 9x19mm Parabellum German pistol cartridge and fired through a blowback action - limited to full-automatic only. The rounds were fed via a 21- or 32-round detachable straight box magazine. The weapon's arrangement was conventional with a standalone pistol grip/trigger unit, receiver, magazine well set ahead of the pistol grip, and barrel. The barrel section was unique in that the designs saw fit to add support for a bayonet. Sighting was through a front and rear iron arrangement. Performance specifications included a rate-of-fire of 500 rounds per minute with a muzzle velocity of 1,280 feet per second. As in the M3 Grease Gun, the wire stock of the FBP was collapsible for compactness. The cocking handle was set along the left side of the receiver within a long-running channel.
In practice, the FBP proved a reliable weapon and well developed for the rigors of military service. Its steel pressing manufacture approach made the weapon suitable for mass production. If there proved one downfall it was in accuracy at range - which can be expected of a "mutt" gun design like the FBP. The original full-automatic-only m/948 was then followed by the improved m/963 which introduced a select-fire capability by incorporating a semi-automatic action. The m/963 was then, itself, followed by the modernized m/973 which introduced a perforated barrel jacket to help with cooling. These were not produced serially. Internally, however, all models of the FBP line remained largely the same.
The Portuguese Army eventually gave up their FBP submachine guns in favor of the Israeli UZI. FBP service therefore spanned from 1948 into the 1980s. The weapon also fell into use by non-military elements in combat across several former Portuguese colonies.