O/U (over/under)
O/U is an abbreviation for over/under, a style of double-barreled shotgun that has gained popularity in America over the past several decades. O/Us have a lean profile similar to that of a level-action rifle, and are generally considered to be more intuitive to aim that SxS shotguns because the barrels are not as off-center from the point-of-aim. The majority of competition shotgun shooters use O/Us.
OAL (overall length)
OAL is an acronym for overall length. OAL is often discussed when examining compact, highly maneuverable shotguns and rifles. OAL is also an important legal consideration, as minimum lengths are required for rifles and shotguns in the United States.
See also: NFA
See also: NFA
objective lens
The objective lens of a telescopic sight is the "front" lens, furthest away from the shooter's eye. A larger objective lens will allow the scope to gather more light, resulting in a brighter, clearer picture. However, a larger objective lens means the scope will be physically larger and heavier, and will also need to be raised higher above the rifle in order to prevent the scope from touching the barrel. It is generally desirable to place the scope as low as possible on the rifle, both to provide a better cheek weld for the shooter and to keep the center axis of the barrel as "true" to the scope's sightline as possible.
Objective lens diameters are usually measured in millimeters. A 25-35mm objective lens is adequate for most shooters, though long-range or low-light enthusiasts may prefer diameters up to 50mm.
Objective lens diameters are usually measured in millimeters. A 25-35mm objective lens is adequate for most shooters, though long-range or low-light enthusiasts may prefer diameters up to 50mm.
OD (olive drab)
OD is an abbreviation for "olive drab", a shade of green common in military uniforms and equipment. Black, OD green, and dark earth are the most common colors used in commercial firearms manufacturing.
off-hand
Off-hand shooting simply means shooting while standing, with no support for the firearm other than the shooter's hands; in other words, shooting off of your hands.
out-of-battery discharge (backfire)
An out-of-battery discharge, sometimes called a backfire, occurs when a cartridge fires before the action is fully closed and locked. Out-of-battery discharges can greatly overstress the firearm's components and potentially harm the shooter or bystanders with combustion gases or flying debris ejected from the firearm's internal components. Out-of-battery discharges are usually caused by firing pins of excessive length or other poorly fitted parts.
A few types of firearms are designed to purposefully fire from a slightly out-of-battery position, but these are rare and must be designed from the ground up. Attempting to convert a firearm to fire in this manner (in the pursuit of fully-automatic fire) can only end in disaster.
A few types of firearms are designed to purposefully fire from a slightly out-of-battery position, but these are rare and must be designed from the ground up. Attempting to convert a firearm to fire in this manner (in the pursuit of fully-automatic fire) can only end in disaster.
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