August 23, 2015


Gun Zero Sight in Distance – The range at which the scope or iron sight of the firearm is zeroed. This is the range where the sight picture shows the actual point of impact. This can vary dramaticly for different type of firearms. Pistols can be zeroed as close as 7 yards, open sight large bore rifles are usually zeroed around 150 yards, while high velocity hunting rifles are zeroed at 250+ yards. The object here is to keep the point of impact of the bullet within + or – 2 to 5 inches at the typical distances that the firearm will be used.

Sight Height Above Bore – Most firearms are provided with sights. These can be iron sights or an optical scope. The sight is attached to the barrel above the firearms bore axis. The typical distance above the bore for iron sights is 0.75 inches, while the typical height for an optical scope is 1.5 inches. These will vary from firearm to firearm and should be measured. Rifles equipped with scopes with large objective lenses may need special high scope rings to raise the scope enough so it does not touch the barrel.

Bullet Diameter – The diameter of the bullet in inches expressed as a fraction. Caliber and bullet diameter are related. For example 50 caliber bullets have a diameter of .50 inches. The firearm industry muddys the water somewhat by giving cartridge caliber names which do not accurately reflect the bullet diameter. For example, the 270 caliber bullet actually has a diameter of .277 inches. Likewise the 243 and 6mm cartridges actually use the same bullet with a diameter of .244 inches. Bullet manufactures web-sites are a good resource if you are in doubt.

Bullet Weight – You are asked to provide the weight weight in grains (not grams). This is a key parameter when calculating bullet energy (remember Force = mass x velocity). The metric cm output options will list bullet weight in grams, even though it was entered in grains.

Muzzle Velocity – The speed of the bullet when it exits the barrel in feet per second (ft per sec). This is an important component for computing drag, which is used to predict the downrange path of the bullet. This app only works for muzzle velocities between 300 and 4800 ft per sec.

G1 Ballistic Coefficient – You will be asked to provide the G1 ballistic Coefficient for the bullet in the cartridge profile. All bullet manufacturers and most ammunition suppliers list the ballistic coefficient on the box. Bullet and ammunition manufacturers use the G1 Ballistic Coefficient – but may omit the G1 descriptor. The “G” number refers the Siacci drag model used to calculate down range bullet performance. Ballistic Coefficient reflects a bullets shape and its diameter to length characteristics. The higher the number, the flatter the bullets trajectory will be (less drop at long distances).

Bullet Drop – appears in the output of some of the ballistic tables. It is the calculation of the bullet drop referenced to the bore of the firearm. Bullet drop is always zero or negative. It is zero when it leaves the muzzle of the firearm, and at all subsequent distances continues to fall as it is affected by gravity. The higher the velocity (shorter bullet time of flight) the less time gravity has to act on it.

Bullet Path – appears in the output of all ballistic tables. It is the calculation of the bullet path with reference to the line of sight (through the sights) rather than the bore of the firearm. The bullet path starts out below the line of sight by the height of the sights above the bore, crosses the line of sight, reaches an apex (called Max Height), before falling and crossing the line of sight again (this is the sight zero distance) and then eventually falls back to earth.

Sectional Density – is a property of a bullet which relates diameter to bullet weight. It is an important factor in estimating bullet depth of penetration in game animals. See Chuck Hawks.

It is listed by bullet manufacturers for their bullets, and it is displayed in the header of the ballistic table of this app.

Bullet Path MOA (Minute of Angle) – is a measure of the bullet path in minutes of angle. A minute of angle is 1.047 inches at 100 yards. It is useful in scope adjustments – both windage and elevation. When sighting in a rifle, many riflemen make their fine scope adjustments while shooting at 100 yard targets where the math is the easiest. If you want your rifle to be 2 inches height at 100 yards (roughly a 250 year zero for a hunting rifle) but it is shooting only 1 inch high, you need to adjust it up 1 MOA. Each scope manufacturer prints the number of clicks per moa on the scope (usually under the adjustment caps). If your scope has quarter inch clicks, then you would adjust it up 4 click to raise the point of impact 1 inch.

Bullet Path Mil (Milliradian) – is a measure of the bullet path in milli-radians. One mil approximates one meter at a distance of one thousand meters, or in other words if a 1 meter long object measures 1 milli-radian then it is 1000 meters away. The danger is that all Mil systems are not the same. In true milli-radians measurements there are 6283 milli-radians in a circle. The NATO simplified this to 6400 mils, and the Russians and other eastern European countries use 6000 mils. This app will use MIL units for true milli-radians. Practically speaking the differences are small – about 2 inches at 1000 yards between true milli-radians and NATO MILs. See mils vs mils vs mils.

Max Height – in the ballistic tables indicates what the maximum height in the bullet path would be if the firearms sight was zeroed at the distance specified. It can be a little confusing since there is a Max Height entry for every distance in the table. Max Height actually occurs in trajectory at slightly over 1/2 of the zero-in range. It is presented to help the user choose the right distance to zero the firearm at.

xWind Drift – means cross (read 90 degrees from the bore axis) wind drift. It is listed for various wind speeds – usually 10, 20, and 30 mph (16, 32, 48 kph). Wind can be the biggest challenge in delivering a bullet to its target. Distance can be measured and accounted for easier than determining the wind speed and direction. To return to our previous example of scope adjustment, consider the case of a 300 yard target with a 30 mph cross wind. If the ballistic tables states that our 300 yard wind drift is 18 inches, then we have to adjust our scope 6 moa (18 inch drift divided by 3 => number of 100 yard increments).

Point Blank Distance – is furthest distance that the firearms sight can be used without compensating for bullet drop. You first have to determine the maximum deviation from zero which is acceptable – say no more than 2.5 inches high and 2.5 inches low. You then select the sight zero in distance which ensures that the bullet path Max height goes not higher than 2.5 inches. You can then use the apps bullet path plot to determine the distance at which the bullet path crosses 2.5 inches on the down path.