The gun comprises a single barrel, receiver, guard and stock, rigidly assembled. Guided in the receiver, parallel to and below the barrel, is the piston, which by its reciprocating motion assures the automatic action of the gun. This reciprocating motion is brought about as follows:—When the bullet in its passage through the bore has passed a port connecting the bore with the gas nozzle, a small portion of the powder gas issues from the nozzle, and impinging in the cup-shaped forward extremity of the piston, throws in the rear. The recoil spring compressed by the piston in its rearward movement now returns the piston to its initial position.
In its movement to the rear the piston, which is accompanied by the breech block, opens the breech, extracts and ejects the spent cartridge case, and brings a fresh cartridge to the loading position. In its forward movement the breech is locked and the shot fired.
Feed.
The cartridges are fed into the gun on flat, tempered steel strips, of 30 rounds each.
A folding barrel rest is provided for steadying the barrel when firing in the prone position which will ordinarily be taken.
For aircraft the gun is mounted on a crutch, and the stock may be replaced by a pistol grip.
The operation of the gun requires but one man, who loads and fires. If a second operator loads the speed of fire is increased from 250 to 400 rounds per minute.
ACTION OF THE MECHANISM
To load, the breech must be opened by means of the cocking handle. As the piston is drawn back, cams on its upper face turn the fermeture nut a partial revolution, disengaging its interrupted threads from corresponding threads on the nose of breech block. This unlocks the breech. Under the action of its spring, the feed piece engages with a shoulder on the piston and locks it in its rearmost position.
To permit the introduction of the feed strip, the feed piece must be lifted to its highest position by pressing up on its lower end. This frees it from the shoulder of the piston, which latter moves forward slightly under the action of its spring and engages with the sear. The feed strip may now be introduced into the guides, cartridge side down, and should be pushed forward until the pawl forming part of the feed spring engages with it, so as to prevent any backward movement.