The University of Cestlep, rather unsurprisingly, is rather fond of ballistics studies. Its scientists have developed the Armour Rating Metric, or ARM, which allows differentiating different types of armour at a glance. A related metric is Penetrative Effect Notation or PEN, which describes how effective a given projectile is at penetrating armour; when PEN is equal to ARM, the chance of penetration is roughly 50%. The ARM system goes as follows:
Unarmoured
ARM 0
Soft (Leather, thick clothing, thin Kevlar, thin steel)
ARM 1
Hard (Medium steel or ceramic plate, thick Kevlar)
ARM 2
8-15mm RHA
ARM 3
15-30mm RHA
ARM 4
30-50mm RHA
ARM 5
50-75mm RHA
ARM 6
75-100mm RHA
ARM 7
100-150mm RHA
ARM 8
150-200mm RHA
ARM 9
200-300mm RHA
ARM 10
300-500mm RHA
ARM 11
500-700mm RHA
ARM 12
700-900mm RHA
ARM 13
900-1200mm RHA
ARM 14
1200mm RHA+
ARM 15