All military armor is defined relative to some specific
threat. The term "bulletproof" is meaningless unless used in relation
to some specific bullet. Armor that would stop a 9mm round cold might
be largely invisible to a 20mm cannon shell. All armor adds weight. To
the extent that weight reduces mobility, armor may make a vehicle more
vulnerable rather than more protected.
Most HMMWV models were not required by the U.S. military to have any
armor protection. Armament carriers and the hard-shelled ambulance models
were designed to provide some degree of protection against fragments and
spent bullets. The "basic" armor package uses a combination of steel,
kevlar and layered polycarbonate windows designed to stop a fragment weighing
17 grains. "Supplemental" armor adds some additional steel to increase
the protection to stop a fragment weighing 44 grains. Neither basic nor
supplemental armor is intended to stop any size bullet at full muzzle
velocity.
The term "up-armored" generally describes some level of protection greater
than basic or supplemental.
