The ASVAB is a series of tests developed by the Department of Defense and is used by the U.S. Army to determine whether you have the mental aptitude to enlist. The ASVAB also helps determine which Military Occupational Specialties (MOS) you qualify for. The ASVAB is required to enlist in the U.S. Army and is valid for two years. The ASVAB may be given in a computerized version at a Military Entrance Processing Station (MEPS) or in a paper version at various Military Entrance Test (MET) sites around the country or at high schools and colleges. 
Because most of the concepts and methods used by the military, and many of its systems are not found in commercial branches, much of the material is researched, designed, developed, and offered for inclusion in arsenals by military science organisations within the overall structure of the military. Military scientists are therefore found to interact with all Arms and Services of the armed forces, and at all levels of the military hierarchy of command.
The AFQT score is a percentile ranking between 1 and 99 that shows where your score is placed in relation to others. For example, if you score a 54 on the AFQT, this means that you scored as well as or better than 54% of other recruits. If you are not satisfied with your score and wish to retake the test to improve your chances at getting selected, you must wait a month before taking it again. You can retake the test as many times as you want, but after 3 attempts the waiting period jumps to 6 months.
As the speed of technological advances accelerated in civilian applications, so too warfare became more industralised. The newly invented machine gun and repeating rifle redefined firepower on the battlefield, and, in part, explains the high casualty rates of the American Civil War. The next breakthrough was the conversion of artillery parks from the muzzle loading guns, to the quicker loading breech loading guns with recoiling barrel that allowed quicker aimed fire and use of a shield. The widespread introduction of low smoke (smokeless) propellant powders since the 1880s also allowed for a great improvement of artillery ranges.
In addition to the AFQT, the military also includes additional composite scores, often called 'line scores.' These scores are based on variations of the ASVAB's ten subtests and are required for certain military careers. For example, an Army Clerical (CL) score is determined by looking at an individual's Word Knowledge (WK), Paragraph Comprehension (PC), Arithmetic Reasoning (AR) and Mathematics Knowledge (MK) scores. Note that the subtests considered for different careers can vary depending on the military branch.
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