Obturation
Of a cartridge, the property of sealing the breech against gas escape through the expansion of the case.
Ogive
1 The curved pointed head of a cylindrical projectile used in rifled firearms. Designed to keep the projectile from tumbling in flight, to reduce air resistance and to increase penetration.
2 Ballistic measurement of the radius of the curve of a bullets nose.
Open sight see Sight
Operating handle
Of a gas or recoil operated semiautomatic weapon, a projection from the operating rod or bolt, by means of which the action can be manually opened.
Operating rod
In gas operated semi and/or fully automatic firearms, a rod extending from the gas piston to the receiver, engaging and operating the breech mechanism.
Operating spring
The spring that closes the action on semiautomatic and automatic weapons. Also called the recoil spring.
Optical sight see Sight, optical
Ordnance
1 Collectively, military supplies, especially weapons, ammunition and related paraphernalia.
2 Branch of the military service concerned with the care and development of weapons.
Orgue, Organ gun
An early firearm, often a hand gun, consisting of several barrels placed side by side and so arranged that all could be fired simultaneously. In the 17th century as many as 160 barrels were set up on one weapon.
Origin of fire, Origin of trajectory
At the instant of firing, the centre of the muzzle of the weapon.
Orthoptic see sight
Outer
Of a rifle target, the space, valued at two points, exterior to the outermost or largest circle.
Oval bore
A method of smooth boring the barrel with a slightly oval cross-section revolving about the bore axis in a manner that causes the bullet in turn to revolve about its own axis.
Over and under gun
A firearm, usually of the hinged frame type, with two or more barrels placed one over the other.