NOTE
The buffer in the M4 should be marked with an “H” for heavy. This is a heavier buffer and aids in reliability by slowing down the cyclic rate. This point is covered in depth in Chapter 8.
Figure 2-1 Direct-Impingement Gas System prior to firing
Figure 2-2 Direct-Impingement Gas System during firing
CHAPTER 3
BALLISTICS
BALLISTICS
The study of the flight characteristics of projectiles (bullets).
TRAJECTORY
The path of a moving body (bullet) while in flight.
HOLDS
(hold-over/hold-under and windage hold left or right)
How far you must place your aiming point over, under, left, or right of the desired point of impact to achieve a hit.
EXTERNAL BALLISTICS
Soldiers must understand the basics of external ballistics so they can make necessary scope adjustments or hold compensations to allow them to hit the target. The external ballistic factors that affect bullet trajectory are:
1.GRAVITY
The force of gravity on a bullet is constant regardless of its weight, shape, or velocity. The longer a bullet is in the air or the greater its angle from the vertical, the more effect gravity will have on its trajectory.
2.MUZZLE VELOCITY
Muzzle velocity is the speed of a bullet as it leaves the barrel, measured in feet per second. Muzzle velocity diminishes as the bullet gets farther away. The bullet reaches its maximum velocity 76 feet from the end of the rifle and slows down from there until it reaches the target.
3.AIR RESISTANCE OR DRAG
Air resistance or drag immediately produces a slowing effect on a bullet.
4.ALTITUDE AND/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 1/2 to 1 minute of angle.
5.TEMPERATURE
Deviation from standard daytime temperature (59 degrees Fahrenheit/15 degrees Celsius) affects bullet trajectory.
A.Cold air is denser than warm air, meaning the bullet must travel through more tightly packed air particles. This causes the bullet to lose velocity, resulting in the impact being lower than the intended point of impact. Cooler air also causes lower chamber pressure, which reduces the initial velocity.
B.Warm or hot temperatures cause the strike of the round to move up.
6.TRAJECTORY
When a projectile exits the muzzle of a rifle, it drops from the line of departure, otherwise known as the center-bore line. As the projectile travels downrange, the velocity is decreased by air drag, giving way to the inevitable force of gravity. This effect creates trajectory.
A.Line of Sight
The line of sight is an imaginary straight line extending from the shooter’s eye through the telescopic sight, or rear and front sights, to the target.
B.Line of Departure
C.The line of departure is an imaginary straight line extendingfrom the center of the barrel to infinity.
D.Zero Range
Zero range is where the projectile intersects the line of sight. It occurs at two points — one on the way up and one on the way down.
E.Apex (maximum ordinate)
Otherwise known as midrange trajectory, the apex is the point where the projectile is at its highest in relation to the line of sight.
F.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
Although gravity and air drag are the only forces that act on the trajectory, other 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.
A.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. Reading and correcting for wind effectively takes practice, especially at longer
B.To shoot accurately in the wind, a shooter must know the wind velocity, wind direction, and the value of deflection at the range at which he is shooting.
8.ANGLES
Firing uphill or downhill at an angle of 45 degrees or more normally causes the bullet to hit high relative to a horizontal trajectory. If the shooter is firing on an angle up or down at a slanted range of 100 yards, the point of impact will be higher than it would be for a level shot of 100 yards. How high depends on the angle.
A.Gravity acts on a bullet only during the horizontal component of its flight (the distance from the shooter 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.
B.The complicating factor in shooting uphill or downhill is that the wind will affect the shot over the entire slant range. 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.
EXAMPLE
Shooter is 300m (984ft) up the side of a ridgeline and wants to engage a target at the base of that ridge with a horizontal distance of 50m. The target’s actual distance from the shooter is slightly over 305m. Shoot the point of aim as though the target is at 50m, using wind calls for a 300m shot. See figure 3-1 for an example.
Figure 3-1
CHANGING LIGHT CONDITIONS
Bright, sunny day vs. cloudy or overcast day, sun in different positions (may affect the zero of the weapon because of the change in the shooter’s perception of the target. This is less of a factor with a red dot sight than with iron sights.) On bright days, shooters tend to hit low, and on cloudy days, they tend to hit high. An easy-to-remember phrase is “lights up, sights up; lights down, sights down.”
CANT OF THE WEAPON (close range vs. long range)
If you cant the weapon while firing (around obstacles or under a vehicle, etc.), you will move the strike of the bullet in the direction of the cant and low (i.e., cant the weapon to the right – the round strikes low and right). This is because when you cant the weapon, the windage becomes the elevation, and the elevation the windage. At close range, it will only be the difference between line of sight and line of bore, but at distance, you will have to adjust for bullet drop as well. At 100m with your rifle canted 90 degrees to the left, you can expect the strike of the round to be 4”-6” left and 4”-6” low. At 100m canted left, aim at the right shoulder. Try not canting your weapon unless you have to, but if necessary, hold in the