Objective Lens - The lens at the front of the telescope, facing the target. It is usually larger in diameter than the ocular lens.
Obturation - The sealing of propellant gases within the chamber of the firearm.
Ocular Lens - The lens at the rear of the telescope, nearest the sniper's eye.
Off-hand position / Off-hand shot - A shooting position in which the shooter stands upright and does not rest the rifle or his arms on or against any object.
Off-hand rifle - A target rifle designed to be held, not rested.
Ogive - The curved or tapering section of a bullets sides, forward of the bearing surface ( i.e. between the cylindrical section and the nose tip. )
Open sight - Rear sight of traditional leaf type with open-topped V-notch or U-notch, as distinct from a scope or aperture ( peep ) sight.
Over bore capacity - Common term to describe cartridges with powder capacity overly large in relation to the bore. Expanding ( burning ) gas can only be forced through a given aperture at a certain rate; thereafter, increasing the amount of gas ( by increasing the amount of powder ) merely raises pressures without raising velocity ( a .300 H&H case necked down to .22 calibre has twice the powder capacity of a .22-250, but produces maximum velocities that are no higher. ) Cartridges which are over bore capacity include some Weatherby calibres, the .264 Win Mag, and even the .25-06.
Over-and-under ( O/U ) - Firearms with two barrels placed one above the other, as distinct from a side-by-side.
Overall Length ( OAL ) - The total length of a cartridge, measured from bullet tip to base of case. ( Click here for picture. )
Overtravel - Continued rearward movement of the trigger after the sear has been released. The better triggers are adjustable for minimum overtravel, to minimize disturbance of the aim while the bullet is still in the barrel.
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