An increase in the hardness of a given thickness of homogeneous armour may result in an increase or in a decrease or in no change at all in resistance to penetration, depending upon the ratio of the thickness of the armour to the calibre of the projectile concerned. Where under-matching projectiles (those whose calibre is less than the thickness of the armour) are concerned resistance to penetration at normal impact increases as hardness increases; where overmatching projectiles (those whose calibre is greater than the thickness of the armour) are concerned, resistance to penetration at normal impact decreases as hardness increases; and where matching projectiles (those whose calibre equals the thickness of the armour) are concerned, little change in resistance to penetration at normal impact occurs over a considerable range in hardness.
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