OBTURATION

The momentary expansion of a cartridge case against chamber walls which minimizes the rearward flow of gases between the case and the chamber wall when the cartridge is fired.

OFFSET MOUNT

A mount for telescopes or iron sights affixed to the gun in such a manner, that the line of sight is to the side and above the receiver or barrel.

OFFSET STOCK

A curved buttstock for use by a shooter who wishes to use the left eye when shooting right handed or vice versa. Also known as a Cross-Over Stock.

OGIVE

The curved portion of a bullet forward of the bearing surface.

OIL DENT

A shallow indentation in a cartridge case, generally caused by excessive oil:
a) present in dies during case manufacture
b) in re-sizing dies during handloading operations
c) in a chamber during firing

OILPROOF

The treatment of a cartridge to minimize the entry of oil or water.

ONE PIECE MOUNT

A mount for a telescopic sight that consists of a one piece base as opposed to a two piece base.

ONE PIECE SHOTSHELL

1. A shotshell component having the body and basewad as a single unit with a metallic cup. Sometimes called Unibody Shell. 2. A complete round of ammunition having the body and basewad as a single unit without a head of a different material.

ONE PIECE STOCK

A full length stock made from a single piece of wood. Includes both the butt and fore-end.