In this case, we have to tackle first the distance that a bullet from the rifle can travel under the water and how deep should a target stay in the water to be protected from rifle bullets. A number of factors such as the angle of firing, shape of the bullet, design of the bullet nose and the velocity of muzzle are to be considered here in order to establish a defense theory.
Soft-nose bullets can hardly penetrate the water compared to jacketed-designed military bullets. If bullets are fired vertically at a steep angle, the bullets can penetrate the water, whereas if the bullets are fired at a shallow angle from the water surface, they tend to drag off in different directions.
In movies, the main character often dived into the water whenever he is being chased by the enemies. How far then should he submerge in water to be safe from bullets?
A bullet travels faster in the air than in water because air is lighter than water. A drag force is more expected in water than in the air that causes the bullet to slow down. If an ordinary bullet with a speed of 300 m. per second is fired into the water, the depth of water where it slows down is only a few meters.
Therefore, a 3 meter submerge from the surface of the water is safe from the bullet. If the person holding the gun fires from the bank of a river, the target only needs to be submerged in 1-2 cm. below the water surface. This is because an ordinary bullet will slip off the surface of the water just like a stone-throwing.
If the bullet is fired from an aircraft, a round-nose bullet can penetrate the water in a steeper range of approximately 30 cm. A sharp-nose military bullet can pierce the water up to 5 cm. only. Bullets fired from pistols have rounded noses that can penetrate the water at a farther distance.
During the war, there were many instances when the soldiers submerged into the water when enemy aircrafts pounded on them on the ground surface. Were they safe in going a few feet underwater?
Many survivors vouched for the safety of submerging into the water when they were attacked by enemy’s aircraft.
An experiment conducted by the Bureau of Ordnance determines the depth needed for protection against .50 caliber bullets and .30 caliber bullets.
If the .50 caliber bullet is fired in a vertical position downward, the critical distance for the bullets to cause harm is 4-5 feet. If the bullets are fired in a slanted angle of 45 to 60 degrees, the danger posed by the bullets is reduced by approximately 2 feet.
When a .30 caliber bullet is fired in a vertical downward direction, full penetration can be observed at 1 foot distance but not more than 2 feet. Therefore, based on this observation, the safe range to submerge in water is 5 feet from a .50 caliber machine gun and 2 feet for .30 caliber machine gun.