Effective Wind - The average of all of the varying winds encountered.

Ejector - Device which expels cartridges or fired cases from a firearm.

Energy (of motion) - Kinetic energy or force carried by a bullet at that point in its trajectory. In common use and popular shooting literature it is expressed in foot-pounds, one ft/lbs being the amount of force required to lift a one-pound weight one foot above the ground. Formula: Energy ( in ft/lbs ) equals bullet weight ( in grains ) multiplied by the velocity ( in feet per second ) squared, divided by 450240. Often wrongly equated with killing power, energy is not a reliable gauge of this, as it does not take into account penetration or bullet performance.

Erosion - The wearing away of the bore due to friction and/or gas cutting.  Normally more pronounced in the throat, and that part of the barrel just beyond.

Exit Pupil - The small circle of light seen coming from the ocular lens of an optical device when held at arm's length.  To get the exit pupil of a telescope, simply divide the size of the objective lens ( in millimeters ) by the power of the scope - for variable powered, take the current setting.  E.g., 8x40 scope, divide 40 by 8, and get 5. The size of the exit pupil will help in determining the effectiveness of the optical device in low-light conditions.  The larger the exit pupil, the better - up to a limit of course. The human eye is normally dilated to about 5mm in normal light, 2-3mm in bright light, and 7-8mm in low light. So you want your exit pupil to be slightly LARGER than pupil of your eye under the conditions you're using the scope in, otherwise blackout will occur.

Express - Term adopted by British gun makers during late 1800s to denote cartridges of unusually high velocity (named after ‘express train’). There were black powder express (BPE) and later nitro express (NE) cartridges. The rifles also became known as express rifles. Today the Americans call the classic British leaf sights ‘express sights’.

Exterior Ballistics - The branch of applied mechanics which relates to the motion of a projectile from the muzzle of a firearm to the target, i.e. the performance of the projectile during flight. Also see Ballistics, Internal ballistics and Terminal ballistics.

Extractor - Device for withdrawing the cartridge or fired case from the chamber. It is normally part of the action, and consists of the ejector claw and ejector pin.

Extractor Groove - Circumferential groove in the head of a rimless, semi-rim or rebated rim case to provide purchase for the extractor claw. ( Click here for picture. )

Eye Relief - The distance that the eye is positioned behind the ocular lens of the telescopic sight. A two-to three-inch distance is average. The sniper adjusts the eye relief to ensure a full field of view. This distance is also necessary to prevent the telescope from striking the sniper's face during recoil.

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