Temperature

Extreme changes in temperature cause fluctuation in the rifle's chamber pressure. This fluctuation is caused by changes in the propellant's temperature. In cold weather, as rifle chamber pressure decreases, the bullet exits the muzzle at a lower velocity, and the bullet impacts the target below the point of aim. In extreme heat, the rifle's chamber pressure increases causing the bullet to exit the muzzle at a higher velocity and impact the target above the point of aim. Hot air is less dense than cool air and provides less resistance to the bullet; this allows the bullet to travel faster and experience less deflection from the wind. Cold air is dense and provides the bullet with more resistance; this causes the bullet to travel slower and experience greater deflection from the wind.

Wind Chart Sniper
Figure 8-4. Windage Click Chart for the Observation Method.

range flag angles

Í 5mph

10mph

1

15 mph

20 iwph

25 »ph

range yards

wind value

wind value

wind value

wind value

wind value

fu lu

half

full

half

full

half

full

half

full

half

200

2

1

1

5

2

6

3

«

Á

300

3

1

6

3

10

s

13

&

16

3

400

A

2

9

4

13

6

18

9

22

11

odd

6

3

12

6

1fl

9

24

12

3d

15

Figure 8-5. Windage Click Chart for the Flag Method.

Figure 8-5. Windage Click Chart for the Flag Method.

Once the rifle is zeroed, a change in temperature of 20 degrees or more can cause the bullet to strike above or below the point of aim. Therefore, if the temperature changes 20 degrees or more, a Marine should re-zero the rifle.

If the rifle is exposed to below freezing temperatures, it should not be brought immediately into a warm location. Condensation may form on and in the rifle, and it may freeze if re-exposed to the cold. Ice that forms inside the rifle may cause it to malfunction. Ice can form on the rear sight aperture due to condensation, making it impossible to acquire sight alignment.

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