JACKET: The envelope enclosing the core of a bullet. The covering, or "skin", of a bullet. Fully jacketed and semi-jacketed bullets have different performance characteristics from simple lead bullets.
JAM: A malfunction that prevents the action from operating. Jams may be caused by faulty or altered parts, ammunition, poor maintenance of the firearm, or improper use of the firearm.
JAMMED: To cause the moving parts or action of a gun to lock into an unworkable position: Jammed the bolt or slide open. Usage "My Glock is jammed."
JFP: Abbreviation for Jacketed Flat Point.
JACKETED FLAT POINT: A designation for a type of ammunition with a bullet constructed like a full metal jacket, however instead of the tip of the bullet being rounded, it ends in a flat plate. Abbreviated JFP.
JACKETED HOLLOW POINT: A designation for a type of ammunition with a bullet constructed of a soft lead core enclosed in a hard metal jacket. The top or point of the bullet has an opening in the jacket, exposing a hollow lead core. On the force of impact, the bullet is forced to open up and expand, resulting in less penetration but greater damage due to the larger diameter of the now expanded bullet. Jacketed Hollow Points are the most commonly used defensive pistol cartridges and are typically very effective. Abbreviated JHP.
JHP: Abbreviation for Jacketed Hollow Point.
JACKETED PRE-FRAGMENTED (JPF): Designation for a type of ammunition with a bullet constructed of a metal jacket as in the Jacketed Hollow Point (JHP), but instead of the bullets core being solid lead, it consists of a number of separate projectiles, such as lead shot (the most common), metal disks, or polymer, (or any combination) compressed into the jacket. The tip of the bullet usually has a polymer "cap" with a round-nose profile to ensure reliable feeding. JPF bullets are usually much lighter than normal bullets, and travel at extreme velocities. Abbreviated JPF.
JACKETED SOFT POINT: A designation for a type of ammunition with a bullet constructed of a soft lead core enclosed in a hard metal jacket. The top or point of the bullet is made of softer metal than the jacket. On the force of impact, the bullet is forced to open up and expand, resulting in less penetration but greater damage due to the larger diameter of the now expanded bullet. Jacketed Soft Points are commonly used for hunting large game and are typically very effective. Abbreviated JSP.
JACKETED TRUNCATED CONE: A designation for a type of ammunition with a bullet constructed like the Jacketed Flat Point - JFP - but the sides of the bullet are straight and end in a flat plate, as opposed to the JFP where the sides are more rounded. The JTC bullet has the profile of a cone with a flat plate at the tip instead of a point.
JDAM (Joint Direct Attack Munition): The Joint Direct Attack Munition — or JDAM — is a low-cost guidance kit that converts existing unguided free-fall bombs into accurately guided "smart" weapons. The JDAM kit consists of a new tail section that contains an Inertial Navigation System/Global Positioning System. In actual use these weapons have proved to be very accurate and highly reliable. They can be accurately delivered in virtually any weather conditions. JDAM can be launched miles from the target and safely rely on the navigation system to update the weapon all the way to impact.
JPF: Abbreviation for Jacketed Pre-Fragmented.
JSP: Abbreviation for Jacketed Soft Point.
JTC: Abbreviation for Jacketed Truncated Cone.
JUMP: The upward and rearward recoil of a firearm when it is fired. It is commonly called recoil or "kick."