1. Gravity - The force of gravity on a bullet is constant regardless of its weight, shape, or velocity. The barrel must be elevated slightly to allow the round to travel farther, creating an arc. The round will travel straight until it slows down and is gradually pulled to the ground by gravity.
2. Muzzle Velocity - Muzzle velocity is the speed of a bullet as it leaves the barrel, measured in feet per second. The bullet begins to slow down as soon as it exits the barrel.
3. Air Resistance (Drag) - Air resistance, or drag, immediately produces a slowing effect on a bullet.
4. Altitude or Air Density - The greater the altitude, the thinner the air and the longer the bullet will travel (with a correspondingly flatter trajectory). Each 5,000-foot elevation will raise the strike of the bullet ½ to 1 minute of angle (MOA).
5. Temperature - Deviation from standard daytime temperature (59 degrees Fahrenheit/15 degrees Celsius) affects bullet trajectory.
• Cold Temperatures - Cold air is denser than warm air; the bullet must travel through more tightly packed air particles. This causes the bullet to lose velocity, causing the bullet to impact lower than intended. Cooler air also causes lower chamber pressure, which reduces the initial velocity.
• Hot Temperatures - Warm or hot temperatures cause the strike of the round to move up.
6. Trajectory - When a projectile exits the barrel, gravity immediately takes effect, causing the bullet to drop from the line of departure, otherwise known as the line of bore. As the projectile travels downrange, air drag decreases the velocity. These effects create the projectile’s trajectory.
a. Line of Sight - The line of sight is an imaginary straight line extending from the firer’s eye through the telescopic sight, or rear and front sight, to the target.
b. Line of Departure - The line of departure is an imaginary straight line extending from the center of the barrel to infinity.
c. Zero Range - Zero range is where the projectile intersects the line of sight. It occurs twice—once on the way up and once on the way down.
d. Apex - Otherwise known as midrange trajectory, the apex is the point where the projectile is at its highest in relation to the line of sight.
e. Bullet Path - The bullet path is the relationship of a projectile and the line of sight at any given range (normally expressed in inches).
7. Wind - External factors influence the trajectory relative to the point of aim, such as wind, altitude, temperature, humidity, and barometric pressure. Wind is by far the most significant. Consider the following effects of wind:
• Because the bullet is moving through the air, the air moves the bullet. Wind deflection is always in the same direction the wind is moving. A wind blowing from the left will move the bullet to the right.
• Deflection decreases as the angle of the wind to the line of flight decreases.Effectively reading and correcting for wind effects takes practice, especially at longer ranges where accuracy in correcting is more critical. To shoot accurately in the wind, a firer must know the wind velocity, the wind direction, and the value of deflection at the range at which he is shooting.
8. Angles - Firing uphill or downhill normally causes the bullet to hit high relative to a horizontal trajectory. If the firer is firing on an angle up or down at a slanted range of 100 meters, the point of impact will be higher than it would be for a level shot of 100 meters. The height depends on the angle.
• Gravity acts on a bullet only during the horizontal component of its flight (the distance from the firer to the target measured as if they were both at the same level). Since the horizontal component will always be less than the slanted range, gravity will not pull the bullet down as far as it would if the range were level.
• Firing uphill or downhill causes the wind to affect the shot over the entire slant range. The firer should aim at the target as if it were 25 meters away and correct for wind as if it were 400 meters away. The correct method for shooting uphill or downhill is to adjust elevation based on the horizontal range and correct for wind deflection based on the slanted range.