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The ZB Co., after a number of years' experience with its light gas-operated machine gun that enjoyed a world-wide reputation for reliability and clean cut design, turned its attention in 1932 to the promotion of a machine gun that operated by recoil forces. In the creation of this new weapon the company advertised that it "made efficient use of all the knowledge and methods which modern technical science puts at the disposal of the successful designer. Painstaking study and research were undertaken to determine kinetic and dynamic conditions in the mechanism of the machine gun. Cinematopographic research was made use of, and radiation of heat from the barrel was the subject of thorough investigation based on the latest data of the science of thermology."

This weapon was given the official designation of ZB-50 and the main change was that the breech mechanism was operated by utilizing the recoil, and the bolt was cushioned by a strong spring so that this heretofore undesirable feature did not influence the accuracy of the weapon when in the act of firing. The bolt assembly was very similar in appearance to the earlier gas-operated models, but in lieu of a gas piston actuating the recoiling parts an accelerator was added that transferred energy during the movement of recoil and at the instant of unlocking exerted its full force on the bolt, speeding it rearward. These features were invented and patented by Anton Marek, one of the noted gun designers in the employ of the ZB plant. The accelerator was in appearance very similar to the well-known Browning type.

This model unlike its magazine-fed predecessors employed metal push-out type links to form a non-disintegrating belt for the purpose of placing cartridges in the feedway. After the discharge of the last round the belt fell out the left side and the bolt and its components were held by a stop in the cocked position. When a loaded belt was inserted, the stops were raised and the bolt moved forward a few thousandths of an inch to be caught by the rear sear. This necessitated a pull of the trigger to release.

The only thing needed for complete disassembly in the field was a loaded cartridge, with the bullet point being used to depress certain spring-loaded detents.

The trigger housing was so designed that it slid on the bottom of the breech casing when released from its spring-loaded detents to cock the mechanism manually. The housing had two small grips on each side that acted as the charging piece. The advantage of this arrangement was that the two grips could be used to cock the piece when it became necessary to load the weapon by hand.

A muzzle booster that trapped the powder gas after the bullet left the barrel was used to increase rate of fire by bringing this pressure to bear on the face of the barrel. This device working in conjunction with the mechanical accelerator gave a cyclic rate of 600 shots a minute. The muzzle booster was also designed so as to act as a Hash hider and front barrel bearing.

The safety catch was of unusual design on this weapon, being so constructed that, when it was on, it held not only the sear locked but also threw the point of the incoming round down at an angle so that if by any chance whatsoever the sear did become disengaged and the bolt go forward the cartridge was in such a position that it would not chamber.

Perfect coordination between the counterrecoiling barrel and the moving belt was assured by a specially constructed piece that permitted the return movement of the bolt only after the barrel had gotten into battery. To fire the ZB-50, the gunner, generally prone, raises a cover in front of the feedway and inserts a loaded cartridge belt from the right side until the first cartridge is under the spring-loaded holding pawl. Then the firing grips are released from their detent and shoved forward until the sear engages the bolt. It is then pulled all the way to the rear. At this point the rear sear engages its recess in back of the bolt, holding the firing mechanism in the retracted position. Rearward motion also compresses the driving spring and moves the belt over one space in the feed-way, thus positioning the first round. When the trigger button is pushed, the sear disengages the bolt which flies forward under the influence of the driving spring energy.

The bolt face, arriving at the rear of the feed, begins the first phase of chambering the round by pushing it out of the link into the guideway that positions the nose of the bullet into the entrance of the chamber. The bolt is made in two pieces. The portion containing the bolt and its rear locking face arrives in battery first with the rear end directly under a locking recess milled into the barrel extension. The second part, which is connected to an extension spring and is held to the rear of the first part by means of the locking lug, is now free to move forward. The angle on the locking lug cams the back end of the bolt up into the locked position. The last forward motion of the bolt assembly's second portion brings the face of the locking lug to bear suddenly on the firing pin housed in the bolt and the weapon is fired.

In the first half inch of recoil, the bolt assembly and barrel are locked to the barrel extension. At the same time they build up tension in the extension-type mainspring. This spring is connected to the accelerator which, upon being activated by the spring, pivots and shoves to the rear the part of the bolt carrying the lug. This unlocks the weapon and also accelerates the already recoiling parts to the rear. The empty case is withdrawn from the chamber and ejected through the bottom of the receiver. When the bolt assembly reaches its rearmost position, counter recoil will begin, repeating the cycle of operation as long as the trigger button is depressed.


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