OPERATING LEVER

On lever action firearms that part operated by the hand to open and close the breech.

OPTICAL SIGHT

A sight containing optical elements which may magnify the target.

ORANGE PEEL

A rough textured finish similar to the skin of an orange, generally referring to certain stock finishes.

ORIFICE

A small hole or vent such as the gas port in the barrel of a gas-operated firearm.

ORIGIN OF TRAJECTORY

When a bullet exits from the barrel of a firearm, the location of the center of the bore at that specific point in time is called the origin of the trajectory.

OUT-OF-BATTERY

The condition of the breeching mechanism of a firearm not being completely locked or completely closed.

OVER AND UNDER

Firearms with two barrels placed one above the other.

OVERALL LENGTH

1. Firearm: The dimension measured parallel to the axis of the bore from the muzzle to a line at right angles to the axis and tangent to the rearmost point of the butt-plate or grip.

2. Ammunition: The greatest dimension of a loaded cartridge, i.e., from face of the head to the tip of the bullet for centerfire or rimfire or to the crimp for shotshells or blanks (not to be confused with the uncrimped length in a shotshell.)

OVER-BORE

A shotgun barrel bore diameter greater than the SAAMI maximum for that gauge. Also called a Baker Bore.