Marines are responsible for maintaining a BZO on their rifles at all times. Many factors influence the BZO of a rifle. If operating in a combat environment, Marines should confirm their BZO as often as possible. Atmospheric conditions, humidity, and temperature can cause BZOs to change on a daily basis. To confirm a BZO, a Marine may begin the zeroing process by using the previously established BZO sight settings rather than placing the sights at initial sight setting. The following factors cause a BZO to be reconfirmed.
a. Maintenance. It is possible for the BZO to change if ordnance personnel perform maintenance on a rifle. If maintenance was performed, it is critical that the rifle be rezeroed as soon as possible.
b. Temperature. An extreme change in temperature (i.e., 20 degrees or more) will cause the elevation BZO to change. Changes in temperature cause chamber pressure to increase when hot and decrease when cold. This causes shots to impact the target high in hot temperatures and low in cold temperatures.
c. Climate. Changing climates (i.e., moving from a dry climate to a tropical climate) can mean changes in air density, moisture content, temperature, or barometric pressure. Any of these elements can affect the rifle's BZO.
d. Ammunition. Inconsistencies in the production of ammunition lots can change a rifle's BZO.
e. Ground Elevation. Drastic changes in ground elevation can create changes in air density, moisture content, temperature, or barometric pressure. Any of these elements can affect the rifle's BZO.
f. Uniform. If Marines zero their rifle in the utility uniform and fire in full battle gear their BZO will change. The wearing of full battle gear changes eye relief, placement of the rifle in the shoulder pocket, and the way the rifle is supported on the handguard. Marines must establish their BZOs while wearing the uniform and equipment they will be wearing while engaging targets.
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