In the 1950's, Pietro Beretta Spa., Italy, developed the first Berettasubmachine gun prototypes, models 6-11. Model 12 was the final production model, and was soon followed by the M12S with differences in the safety and other mechanics.
The Model 3 weighs 3.48 kilograms empty (about 3.820 kg loaded) and is 660 millimeters in length with stock extended (418 mm when retracted). Its short length is achieved by use of a barrel recessed into the bolt head, known as a telescoping bolt. This reduces length without reducing barrel length or bolt weight.
It fires from an open bolt and has a cyclic rate of fire of 550 shots per minute. Its muzzle velocity is 380 meters per second. It is accurate to 200 meters.
The barrel and rifling are chromium-plated to prevent fouling. The bolt housing has grooves to allow bolt movement, even in extremely adverse conditions such as exposure to mud, dust, or sand. The exterior surfaces of the firearm are finished with epoxy resin coating for protection against corrosion and damage.
The weapon has a selective-fire option allowing a choice of single shot or fully automatic fire.
The weapon has 3 safeties: a manual safety which blocks the trigger; an automatic safety on the rear grip which immobilizes the trigger and blocks the bolt in a closed position; and a safety on the cocking handle locking the bolt in case it does not retract sufficiently.
The weapon is provided with a front sight (adjustable for elevation and windage) and a rear sight with a 2-position flip aperture (up to 100 m and up to 200 m).
The gun is equipped with a side folding stock, but is also seen rarely with a fixed stock.
20, 30, and 40-round box magazines were available for the original Model 12, which was chambered for the 9 mm Parabellum cartridge.
The Model 12 was redesigned as the Beretta Model 12s in 1978 and was styled as "the Ultimate Submachine Gun" when released. The Model 12s uses a 32-round box magazine, and is chambered for the 9 mm x 19 NATO cartridge. It utilizes the blowback principle, and is capable of either semi- or full-automatic fire.
A novel feature is the grip safety, which locks the trigger and the bolt in the closed position, thus safeguarding against accidental firing if the grip is not held firmly or if the gun is dropped. The safety and fire-selector switch, which in the original Model 12 were two separate push-pin button (with the fire-selector being a button that activated single-fire or burst fire whether it was pushed on the right side or the left side) have been re-engineered in a modern lever-type selector with three positions (S for "Sicura" or Safety, 1 for Single-fire, R for "Raffica" or Burst fire). The fixed firing pin on the face of the bolt can strike the primer only when the cartridge is chambered fully, and this also avoids accidental firing, according to its designers.
The PM12S was also designed with easy field-stripping and reassembly in mind, which has been simplified and can be accomplished without tools. It can be equipped with a suppressor, but this requires a slight modification of the barrel by a competent gunsmith.
Minus the suppressor and other optional features, the Beretta PM12S is made up of 84 discrete components.
The current version of the Beretta Model 12, called the PM12-S2, spawned from the adoption of the PM12S by the FrenchGendarmerie Nationale, to be assembled under license at the MAS armaments factory in Saint-Etienne from Italian-made parts. The Gendarmerie asked for a modification, in the form of a further safety device, which allowed both to keep the bolt of the weapon in half-cocked position and acted as an interceptor preventing accidental fire should the bolt or the firing pin suddenly disengage. The so-modified Model 12s were known in French service as the PM12-SD with SD standing for Demi-arme, or Half-cock. In the mid-1990s, this modification was implemented in all the Model 12 sub-machineguns manufactured by Beretta as a standard factory feature, and the denomination of the sub-machine gun changed to PM12-S2; this is the only Model 12 variant currently manufactured by Beretta.