Freebore

  Freebore (cross-section of 4 below)
  Rifling (cross-section of 5 below)
A = land diameter, B = groove diameter
Reversed schematic of chamber illustrated in animation below: 1,2 & 3 = chamber
Insertion and removal of cartridges from the chamber of a rifle barrel

A firearm freebore is a portion of the gun barrel immediately forward of the chamber from which the rifling has been reamed away.[1] Freebore is the smoothbore part of the barrel the bullet travels through before being engaged in the barrel's reduced diameter rifling lands, therefore it is "free" from rifling grip.[2]

Location and dimensions

The chamber is the rearmost portion of a rifled barrel enlarged to hold a cartridge containing a bullet. The bullet typically extends forward from the leading edge of the cartridge. The portion of the barrel forward of the chamber tapers into the rifling as a throat. Each barrel is manufactured with a section of grove diameter (or larger) freebore to allow the cartridge to be loaded into the chamber without the resistance encountered if the bullet were forced into the rifling. Freebore diameter in excess of groove diameter for easy loading allows potential bullet misalignment. Increasing freebore length permits bullets to extend further out of the cartridge case to increase space for propellant within the case.[3] Rifling lands taper from groove diameter to land diameter in a leade forward of the freebore.[4] Increasing freebore length delays the onset of resistance from rotational inertia when the bullet engages rifling, and separates that resistance from the internal ballistics transition from static friction to sliding friction. Dimensions of freebore length and diameter may gradually increase if hot powder gas melts the interior barrel surface when the rifle is fired.

Sources

  1. ↑ "Internal Ballistics". Hornady. Retrieved 21 June 2018.
  2. ↑ "Rifle Barrel Free-Bore". Gunners Den. Retrieved 21 June 2018.
  3. ↑ Mann, Richard. "OAL and Free-Bore". Wolfe Publishing Company. Retrieved 21 June 2018.
  4. ↑ Cali, Damon. "An Introduction to Rifle Chambers". Bison Ballistics. Retrieved 21 June 2018.
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