Winchester introduced the shotgun in 1954 as its Model 50 Automatic Shotgun in 12 and 20 gauge. The shotgun featured an Inertia Operated Bolt Lock designed by Williams. The bolt block and cartridge sat within a large floating chamber. When the gun was fired, the gas forward of the floating chamber forced the chamber to the rear approximately 1/10th of an inch in a short stroke that generated the energy necessary for the bolt block to disengage the rear of the floating chamber and operate the semi-automatic action. This was the first semi-automatic shotgun with a fixed nonrecoiling barrel. In 1960 Winchester introduced its Model 59 Automatic Shotgun, which also utilized the Williams design from the Model 50. This model featured the Winlite 'glass' barrel; it was a thin tube wrapped with microfilament glass fibre. This model could be had with the first ever screw-in Versalite choke tubes.