A weapon with a telescoping bolt (also known as an overhung bolt) is one with a bolt

Bolt (firearm)

A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the propellant burns.In manually-operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas...

 which telescope

Telescoping (mechanics)

Telescoping in mechanics describes the movement of one part sliding out from another, lengthening an object from its rest state. In modern equipment, this is often done by hydraulics....

s over, that is, wraps around and past, the breech end of the barrel. This feature reduces the required length of a weapon such as a submachine gun

Submachine gun

A submachine gun is an automatic carbine, designed to fire pistol cartridges. It combines the automatic fire of a machine gun with the cartridge of a pistol. The submachine gun was invented during World War I , but the apex of its use was during World War II when millions of the weapon type were...

 significantly, and it allows rifle designs to be balanced around the pistol grip in a way that gives "pointability" similar to a pistol's.

While it would be simpler and easier to shorten the bolt to fit completely behind the breech, the bolt must have a certain amount of mass in order to operate reliably with a given caliber. The telescoping bolt moves some of that mass forward of the bolt face, resulting in a bolt which is longer overall, but is shorter behind the bolt face.

Though technically a different, distinct concept, nearly all telescoping bolt submachineguns do use an ammunition magazine located in the pistol grip which is used to hold the weapon.

History

The telescoping bolt concept first appeared on semi-automatic

Semi-automatic firearm

A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...

 handguns designed by John Browning

John Browning

John Moses Browning , born in Ogden, Utah, was an American firearms designer who developed many varieties of military and civilian firearms, cartridges, and gun mechanisms, many of which are still in use around the world...

, such as the M1911

M1911

The M1911 is a single-action, semi-automatic, magazine-fed, and recoil-operated handgun chambered for the .45 ACP cartridge. John M. Browning designed the firearm which was the standard-issue side arm for the United States armed forces from 1911 to 1985. The M1911 is still carried by some U.S....

-series pistol produced for the U.S. Army. These pistols feature a slide, which acts as both a barrel shroud

Barrel shroud

A barrel shroud is a covering attached to the barrel of a firearm, that partially or completely encircles the barrel which prevents operators from injuring themselves on a hot barrel. Slides, extensions of the stock that do not fully encircle the barrel, and the receiver of a firearm itself are...

 and the bolt. This allowed Browning to design compact pistols for high-powered cartridges.

The first production model submachinegun using the telescoping bolt concept was the Czechslovakian Cz 23 aka Sa.23 or vz.48b series

CZ Model 25

The CZ Model 25 was perhaps the best known of a series of Czechoslovak designed submachine guns introduced in 1948. There were four generally very similar submachine guns in this series: the Sa 23, Sa 24, Sa 25, and Sa 26...

, first produced in 1948. These submachineguns use a cylindrical telescoping bolt with centered barrel. While widely exported in the third world, the Cz 23 series were not well known in the west.
The popularly well known first example was the Uzi submachine gun

Uzi submachine gun

The Uzi is a family of Israeli open bolt, blowback-operated submachine guns. Smaller variants are considered to be machine pistols. The Uzi was one of the first weapons to use a telescoping bolt design which allows for the magazine to be housed in the pistol grip for a shorter weapon.The first Uzi...

, designed in Israel by a designer inspired by the Cz 23 series. This gun has become probably the best known submachinegun of all time. It uses a rectangular bolt, with a barrel which is offset toward the bottom of the bolt. This configuration places the axis of recoil

Recoil

Recoil is the backward momentum of a gun when it is discharged. In technical terms, the recoil caused by the gun exactly balances the forward momentum of the projectile and exhaust gasses, according to Newton's third law...

 lower, increasing the controllability of the weapon in full-automatic fire. The Uzi was designed in 1948 after first models of the Cz 23 were seen, and entered service in 1951.

Subsequently, the telescoping bolt has been used in a wide variety of submachinegun designs.

Comparisons

As the image diagrams demonstrate, the basic concept of telescoping bolt and magazine well in handgrip produce significantly more compact weapons. These diagrams show the 1938 design (1939 service) MP-40 submachinegun, which is 630 mm (25 inch) long, weighed around 4 kilograms (8.8 pounds) and has a 251 mm (10 inch) barrel, and the similar materials and production technology Uzi, a 1948 design (1951 service), which is 470 mm (19 inch) long with stock folded and weighed 3.5 kilograms (7.7 pounds) and uses a 251 mm (10 inch) barrel.

Utilizing nothing more than a configuration change, and the same materials and fabrication technologies, the Uzi is 500 grams (1 pound) lighter and 160 mm (6 inches) shorter.

Examples

• Cz 23 series
  CZ Model 25
  The CZ Model 25 was perhaps the best known of a series of Czechoslovak designed submachine guns introduced in 1948. There were four generally very similar submachine guns in this series: the Sa 23, Sa 24, Sa 25, and Sa 26...
  
  
• Uzi
  Uzi submachine gun
  The Uzi is a family of Israeli open bolt, blowback-operated submachine guns. Smaller variants are considered to be machine pistols. The Uzi was one of the first weapons to use a telescoping bolt design which allows for the magazine to be housed in the pistol grip for a shorter weapon.The first Uzi...
  
  
• Ingram MAC-10
  MAC-10
  The MAC-10 is a highly compact, blowback operated machine pistol developed by Gordon B. Ingram in 1964.-Design:The M-10 was built predominantly from steel stampings...
  
   and MAC-11
  MAC-11
  The Ingram MAC-11 is a machine pistol developed by American gun designer Gordon Ingram at the Military Armament Corporation during the 1970s...
  
  
• Steyr MPi 69
  Steyr MPi 69
  The Steyr MPi 69 is a 9 mm submachine gun of the late 20th century made by the Austrian firm Steyr.-Characteristics:The MPi 69 is shaped much like other telescoping bolt submachineguns, such as the MAC 10 or Uzi...