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Evaluating Ballistic Armour
Meaningful and accurate evaluation of ballistic armour products can only be obtained by controlled testing using standard procedures. Of the wide range of local, national and international ballistic testing standards, the most widely used standards for lightweight armour systems, such as body armour, helmets and shields, are those published by the United States Department of Justice. The NIJ tests, as they are commonly known, define a series of ballistic performance levels.
Armours are classified according to the illustrated Performance levels and detailed individual specifications for body armour, ballistic helmets and protective materials define precise performance requirements for each performance level. In addition to stating the ammunition type and impact velocity, requirements for the number of impacts, angle of impact, minimum separation between impacts, maximum allowable deformation of the armour and methods of armour support for test samples are defined. The maximum deformation of a body armour during ballistic impact is determined using a deformable block of plastilina, a plasticine-like substance, placed directly behind the garment during testing.
 Deformation of the garment causes an indentation in the plastilina block. The depth of indentation is measured and taken as an indication of the level of blunt trauma which may be inflicted on the wearer of the body armour. The indentation depth is measured as indicated in Fig 1.
Body armour test methods usually require a given ballistic threat to be defeated by the armour. This means that the bullet must be stopped within the armour. Many ballistic material specifications, however, use a term known as V50 to define the ballistic performance. For any given ballistic projectile, fired at a given armour, the V50 is the impact velocity at which there is a 50% probability that the projectile will perforate the armour. At impact velocities above the V50 it is unlikely that the armour will defeat the threat, whereas at impact velocities below the V50, the projectile is likely to be stopped by the armour. This is illustrated in the graph above, Fig 2.
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NP's Ballistic Test Range 
 Armour development and product assurance testing is carried out using NP’s in-house ballistic test range. Common ballistic test standards are used as a matter of routine and special test methods can be adopted to suit individual customer requirements. In addition to this, independent tests are carried out to certify the performance of NP’s armours by laboratories in the UK, Europe and the USA.
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Evaluating Ballistic Armour 
