The is a rundown of terminology and basic information you will see in discussions pertaining to ballistics. It may be helpful if you have never taken an interest in ballistics before, or are a new shooter. Truth be told, most shooters are more or less unaware of this information, or are only vaguely aware of it. Accuracy: Accuracy is expressed in group size at a specific distance, normally 5 shot groups at 100 yards. Group size is measured in Minutes of Angle (MOA), which is an angular measurement, meaning the diameter of 1 MOA increases with range. For example, 1 MOA at 100 yards is approximately 1'', and 1 MOA at 1,000 yards is approximately 10''. In other words, group size will open up with increased range, regardless of the accuracy of the rifle. Therefore, if a shooter says his rifle shoots 1 MOA at 300 yards, he means his group size at that range will be approximately 3''. There is also a similar metric system of angular measurement, measured in Milliradians (mils), which is approximately 10 centimeters at 100 meters. Thus .1 Mil is 1 centimeter at 100 meters, which would make 1 MOA roughly equivalent to .3 Mils, given that there are approximately three centimeters in one inch. Velocity: Bullet velocity is measured in feet per second (fps) or meters per second (mps). Published velocities are always given at the muzzle (Muzzle Velocity), which can be used to calculate velocity at any range if other values are known, such as the bullet's characteristics and atmospheric conditions. Bullet weight (mass): Bullets are almost always measured in grains (gr), except in metric calculations, in which case grams are used. This is important if calculating energy or velocity, so make sure all your units match. 1 gram is approximately 15.5 grains. Kinetic engergy: This is the total energy of a bullet in flight, with the bullet's mass combined with its velocity at any given point in its trajectory. Bullets reach their peak energy upon leaving the barrel and lose energy over their trajectory as velocity is lost. The equation to calculate kinetic energy is: E=1/2MV^2. Published energy ratings are always given at the muzzle (Muzzle Energy) in foot pounds (ft-lbs) or Joules (J). Energy can be calculated at any range if the velocity is known, along with the mass of the bullet. Ballistic coefficient: The ballistic coefficient (BC) is essentially a measure of how aerodynamic any given bullet is, or rather how well it overcomes air resistance. It is essentially the mass of the bullet divided by its cross section, with its shape taken into account. The higher the BC value, the more efficiently the bullet cuts through the air. Sectional density: Sectional density is closely related to ballistic coefficient, in that bullets with high BC values will typically also have good sectional density. For purposes of small arms ballistics, sectional density is a predictor of how well a bullet will penetrate a given medium. Hydrostatic shock: Also known as "hydraulic shock," this is the general term used to describe the "remote wounding effects" produced by a bullet striking tissue. In general, it is thought of as a pressure wave, causing the tissue around the bullet to be pushed outward, which can cause wounds greater than the diameter of the bullet when the force of this pressure exceeds the elasticity of the tissue in question. The pressure wave moving through the tissue can also cause damage to the nervous system. Hydrostatic shock is poorly understood and much debated, even amongst the experts. Various armies and medical professionals have studied the effect with different conclusions. It is from this phenomenon that we derive terms such as stopping and/or knockdown power, which don't have any specific definition and are for the most part useless and confusing. In any case, hydrostatic shock is directly linked to kinetic energy, in that the higher the energy the more likely it is that any given bullet will damage tissues and systems with which it doesn't come into direct contact. FBI protocol: Since the 1990's, the FBI has set the testing standards for all law enforcement ammunition, and by extension, dominates civilian self defense ammunition as well. In essence, a bullet must penetrate 12-18'' of gelatin, even if fired through light barriers like heavy clothing, auto glass, plywood, sheet metal, and drywall. A bullet is considered successful based on consistency and how close it comes to the 18'' maximum without exceeding it. Or in other words, the perfect bullet would always come to rest at exactly 18'', regardless of any barrier tested. A bullet penetrating less than 12'' or greater than 18'' is considered a failure, and would be rejected for duty use. Civilian self defense ammunition is often marketed from a standpoint of its ability to pass the FBI testing protocols. 10% ballistic gelatin: This is the standard medium for testing bullets. 10% gelatin mimics the composition of human tissue, and is therefore a good predictor of how a bullet might behave in an actual target, in terms of expansion and fragmentation. It is not, however, a one to one comparison with human physiology, but rather a consistent medium to compare one bullet to another. Bone and variations in tissue in actual human targets can and will have unpredictable effects on bullets. Therefore, you cannot look at a block of ballistic gel and assume that's what the wound in an actual target would look like. Permanent wound channel/cavity (crush cavity) vs. temporary cavity (stretch cavity): The permanent wound channel is the actual cavity created by the bullet as it travels through ballistic gel, which does not go away when the gel settles down. The stretch cavity is the expansion that results from the pressure wave, which then collapses and rebounds on itself due to the elasticity of the gel.