+p and +p+
Designation of a cartridge's chamber pressure level in relation to standard SAAMI specifications.  See Plus-P Facts for more information.

BJHP
Brass Jacket Hollow Point.  A JHP using brass rather than copper for the jacket material.  Most commonly refers to Remington Golden Saber bullets.

bullet
Projectile which is fired by a gun.  Commonly made of lead, but sometimes steel, tungsten, aluminum, or plastic.  May or may not be jacketed in copper, brass, steel, or aluminum.  Contrary to popular slang, it is incorrect to call a cartridge a bullet.

caliber
(1)  The diameter of a bullet measured in inches ... e.g., a 0.355" caliber bullet. 
(2)  Formal designation of a unique cartridge specification ... e.g., "9x19mm" or ".45 ACP" or "357SIG."  Often, the caliber (designation) of a cartridge is not the same as the caliber (diameter) of the bullet used by that caliber (designation) ... e.g., a .38 Special cartridge uses a 0.357" caliber bullet.

cartridge
A complete, self-contained round of ammunition including a bullet, primer, and gunpowder held in place by a cartridge case.

Cast
Bullets made by casting are said to be cast.  Casting is a process for forming lead or lead alloy bullets where molten metal is poured into moulds and allowed to cool.  Cast bullets are often made in alloys containing small amounts of other metals (e.g., antimony and/or tin) which make the bullets harder than pure lead bullets.

chamber pressure
Pressure inside a barrel's chamber when a gun is fired.  Usually measured in pounds per square inch (psi) and thousand pounds per square inch (kpsi).   Each caliber has a SAAMI-specified maximum chamber pressure ... e.g., 35kpsi for 9x19mm and .40 S&W, or 38.5kpsi for 9x19mm +p.

Extreme Spread
See group.

feet-per-second
Unit of velocity frequently used to measure the speed of a bullet.   One-thousand feet per second is equal to approximately 680 miles per hour.   Frequently referred to by the shorthand "fps" ... e.g., 1000fps = 681.8mph.

FMJ
Full Metal Jacket.  Designation for a bullet which has a metal jacket (usually copper gilding, but sometimes steel, brass, or aluminum) surrounding the core (usually lead, but sometimes steel, tungsen, aluminum, or plastic).  The bullet is designed to remain intact without deformation or expansion when striking a target.   Most FMJ bullets actually have a base with exposed lead, but this is not visible when the bullet is seated in a cartridge case (see TMJ).  The solid nose of an FMJ enhances feed reliability.  FMJ ammunition is used by the military and is popular among private citizens for practice and plinking.

foot-pounds
Unit of energy equal to approximately 1.356 joules or 0.324 calories.   Often used to measure muzzle energy.  Sometimes seen as "ft-lbs."

Gas Check
A thin copper cup affixed to the base of lead bullets to reduce leading of the barrel by keeping burning propellant gases from melting the base of the bullet when it obturates.  Typically used for unjacketed lead pistol bullets in magnum cartridges.

gelatin
Material used to test internal ballistics (penetration and expansion, as well as permanent wound channel and temporary stretch cavity ... see Choosing Defensive Ammo for more details) of a bullet.  Proper testing requires precisely calibrated gelatin tested by firing a small standardized projectile (BB) and measuring the depty of penetration.  For more details, see FBI Ballistic Test Protocol.

grain
Unit of weight commonly used to measure bullets and powder charges.   There are 7,000 grains in a pound.

group
Set of bullet holes on a target.  Usually measured in inches, this is used to indicate accuracy.  For handgun accuracy, the standard is to shoot five rounds at twenty-five yards and then measure the size of the group from the center of the two holes farthest from one another.  Because it is difficult to determine the exact center of the bullet hole, other methods are used (outer edge of one hole to the inner edge of the other hole; or outer edge to outer edge, then subtract the caliber of the bullet).  This is called "extreme spread" and is by far the most common method of measuring accuracy.  See also Mean Radius.

LRN
See RNL.

JHP
Jacketed Hollow Point.  Designation for a bullet which has a hollow cavity in the nose.  The bullet is designed to expand rapidly when it contacts a fluid medium.  This has two primary effects: (1) the bullet will make a wider hole in the target, and (2) the bullet will meet more resistance moving through the target and therefore penetrate less deeply.  JHP ammunition is used by most police departments and is also the choice of most private citizens for personal or home defense.

Mean Radius
Method used by U.S. Military and NATO to measure accuracy.  A group is fired and the exact center of the group is determined.   Then each bullet hole is measured to see how distant it is from the center.   The average of these distances is the Mean Radius.  Much more complicated than the Extreme Spread method, Mean Radius gives a much better indication of accuracy because it takes into account every shot fired rather than only the two farthest outliers.

meplat
The flat or blunt end of the bullet nose.  In JHP bullets, the meplat defines the diameter of the hollowpoint opening.  Frequently and incorrectly refered to as "ogive."

muzzle energy (ME)
Measurement of the energy of a bullet (in foot-pounds) as it leaves the muzzle.  Energy is a function of bullet velocity and bullet weight.   The shorthand equation to determine muzzle energy is (bullet weight in grains) times (bullet velocity in feet per second) times (bullet velocity in feet per second) divided by 450,240.  In physics, energy is the ability or capacity to do work or to produce change.

muzzle velocity (MV)
Speed of a bullet as it leaves the muzzle, usually measured in feet-per-second.

Obturation
The process of sealing the bore to prevent the escape of gas.  Pistol bullets typically obturate when fired by expanding at the base.

ogive
Portion of a bullet where the bearing surface ends and the point begins.

OSS
One Shot Stop.  A supposedly scientific measurement of ammunition effectiveness in terms of "stopping power."  For more information, see The Myth of One Shot Stops.

RN or RNL
Round Nose (Lead).  Designation for a raw lead bullet with a round nose profile.  Modern usage is primarily limited to practice ammunition for revolvers.

SAAMI
The Sporting Arms and Ammunition Manufacturers' Institute.  Made up of major firearms and ammunition manufacturers, produces voluntary specifications related to firearm and ammunition quality and safety.  With regards to handgun ammunition, SAAMI produces the specifications on overall cartridge and case length, bullet diameter, chamber pressure, etc.  SAAMI has its own web site.

Stopping Power
See Choosing Defensive Ammo.

Swaged
Bullets formed when lead wire is mechanically squeezed into shape are said to be swaged.

SWC
Semi-Wadcutter.  Designation for a bullet, usually raw lead with no jacket, which has a truncated cone shape to the nose.  Often used for target shooting.

TMJ
Total Metal Jacket.  A bullet with a completely enclosed core.   Similar to the FMJ, but without the exposed base.  This term is used by Blount (Speer and CCI ammo brands) to identify bullets which have an electroplated jacket as opposed to a normally applied jacket.