A breakdown of the M855 round

A breakdown of the M855 round

By RICHARD LARDNER Associated Press Writer The M855 round used in the M4 and M16 rifles is also referred to as 5.56 mm. That metric measurement is the diameter of the inside of the rifle barrel. The larger M80 fired by the M14 is a 7.62 mm round. Both types are manufactured at Lake City Army Ammunition Plant near Kansas City, Mo., and by General Dynamics in St. Petersburg, Fla. An M855 round has multiple parts. The bullet, or projectile, is made by first inserting a carbon steel penetrator into a copper jacket. A lead slug is then loaded behind the steel. The bullet is weighed in grains. One grain equals one-seven thousandth of a pound. The bullet in most M855 rounds weighs 62 grains, although there are 77 grain versions, too. By comparison, the bullet in a 7.62 mm M80 round weighs about 146 grains. Below the M855 bullet is 26 grams of WC844 gun powder, also called propellant. Under that is a primer. Bullet, power and primer are housed in a brass case. When the rifle’s firing pin hits the primer, hot gases moves through a vent hole in the bottom of the case and into the propellant. Pressure builds and the bullet is launched out of the rifle barrel at about 3,000 feet per second. Published in The Messenger 5.30.08

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