Deburring tool - The deburring tool removes burrs from the inside and outside ( chamfer ) of newly trimmed case mouths.

Deflection - The change in the path of the bullet due to wind or passing through a medium.

Deprime, Decap - The process of removing the spent primer from a cartridge case for reloading.  This is most often accomplished by the decapping pin in a die during the resizing operation of reloading.

Detailed Search - A systematic observation of a target area in detail, using overlapping observation in a 180 degree area, 50 meters in depth, starting in and working away from the observer.

Deterrent Coating - Varnish-like coating on powder granules to inhibit combustion, for controlling burning rate, to ensure that the power peak is not reached too early inside the chamber.

Detonation - In rifle terms, a near-instantaneous explosion known to occasionally (but not predictably) occur when reduced loads of slow burning powder are used in large capacity cases, and which has totally destroyed rifles.

Dies - In reloading, tools for resizing, full length of neck only, and depriming cases, and seating bullets.
           In manufacturing, it swages bullets or cores, extrudes lead wire or draws jackets.

Double-barrel - Two barrels in a firearm mounted to one frame. Can be vertically or horizontally aligned. See over-under and side-by-side ).

Double Base Powder - Smokeless propellant containing nitro-glycerine in addition to the basic nitro-cellulose ingredient.

Double trigger - A term used for firearms having two barrels and a separate trigger for the discharge of each. The two trigger are normally placed one after the other, with the understanding that the back trigger must be used first, since squeezing the front trigger might set of the back trigger as well, resulting in both shots going off simultaneously.

Drag  - The aerodynamic resistance to a bullet's flight.

Dram equivalent - A term used to indicate the approximate velocity of a shot charge by a comparison.

Draw mark - A straight scratch lengthwise on the case due to some foreign substance in the drawing dies.

Drift - Lateral movement of a bullet away from the line of bore, caused by its rotation on its own axis, and always in the direction of the rifling twist (compensated for by sight adjustment). See Magnus effect. Also used to describe the influence of wind on a bullet’s flight path.

Drop - Drop is the vertical distance from the line of sight to the comb, Monte Carlo or heel of the stock. It is measured from an extension of a straight line drawn from the base of the front sight bead across the top surface of the open rear sight adjacent to the notch.  When making a custom stock, the drop is specifically adapted to the shooter’s body and shooting style.  The drops for target rifles are usually measured from the centreline of the bore.

Drop Safety - Prevents the firing pin from hitting the primer unless the trigger is pulled.

Dropped primer - See Blown primer.

Dry firing - Aiming and firing the weapon without live ammunition. This is an excellent technique to improve marksmanship skills and does not cause any damage to a center-fire firearm. It is best done with an expended case in the chamber to cushion the firing pin's fall.

Dud - In small arms jargon, a cartridge that misfired. Also see Hangfire.

Dummy Round - Cartridge containing a bullet but no propellant, and an inert primer. Used for safe demonstration of loading procedure, testing of rifle actions, etc. Usually identifiable by hole drilled through case wall.

Duplex load - The use of two different powders in loading the same cartridge.  NOTE  This is not something that should be undertaken by the handloader at home - it can only be achieved safely under laboratory conditions.

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