The Colt Model 1911 semi-automatic pistol is the most popular semi-automatic pistol in U.S. military history. The M1911 family of .45-caliber pistols, invented by John M.Browning and placed into service in 1911, was the U.S. armed forces standard-issue sidearm until 1985, when it was replaced by the 9mm Beretta M9 pistol. In total, about 2.7 million M1911 and M1911A1 pistols were produced. The Colt Series 70 guns were introduced in 1971, which was the first design change since 1926. The Colt Series 80 pistols debuted in 1983.
In an attempt to improve the accuracy of the standard-issue weapons, the Mark IV Series 70 used a redesigned barrel bushing and barrel. The new collet bushing used four steel fingers that gripped the larger muzzle end of the barrel as the gun returned to battery (as the slide moved forward after ejecting a spent shell casing). This design improved the weapon's accuracy by tightening the fit of the barrel and bushing without Colt's having to spend extra time hand-fitting the old solid bushing to the barrel and slide for each individual gun during the manufacturing process.
When it worked, the new design worked well. However, if the inside diameter of the slide was too small, this would cause the collet fingers to get wedged between the barrel and the slide, buckling them and causing them to eventually break. If the slide's fit was too loose, the collet bushing couldn't grip the barrel tightly enough and the barrel would be less stable, causing the pistol to lose accuracy. By about 1988, the collet bushing had begun to be eliminated from the production line, replaced by the old solid barrel bushing.
The Colt Government Model and Gold Cup model M1911s were produced with the Series 70 designation. The shorter-barreled Colt Commander model kept the solid bushing design, so it never had a Series 70 designation.
Firing Pin Block Safety
The Colt Series 80, introduced in 1983, was formally known as the Colt MK IV Series 80 line of pistols. This model instituted a big change to the 1911 design: a new firing pin block safety system.
This system featured an arrangement of internal levers and a plunger designed to ensure the firing pin was blocked until the trigger was pulled. The result was the gun could not accidentally discharge if the weapon was dropped on a hard surface.
Colt's entire line of 1911-style pistols were switched to the new design in 1983, so they all bore the Series 80 designation --- even the Commander and Officer's ACP models. Between 1983 and 1988, Government and Gold Cup models of Series 80 guns contained Series 70-type barrels and bushings.
Hammer Half-Cock Notch
The hammer's half-cock notch is designed to stop the hammer's fall if your thumb should slip off the hammer while in the process of manually cocking the pistol; this way, the gun will not accidentally discharge. The Series 80 pistols were redesigned so the notch was changed to a flat shelf shape instead of a hook, which could break and allow the hammer to fall anyway.
The half-cock notch was also relocated closer to the hammer's full decocked (at rest) position. This way, even if you pulled the trigger while the hammer was half-cocked, the hammer's fall couldn't impact the firing pin with enough force to set off the primer in the chambered shell (it couldn't "go off half-cocked").
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