Abstract : A two-phase study was conducted to determine the efficacy of MIL-STD-662E in ballistic protection limit determination for advanced lightweight armor technologies. The first-phase results demonstrated an unacceptable variability in limit estimates across sites. It was concluded that more stringent controls in implementing MIL-STD-662E, particularly as to test velocity selection, would be required to achieve adequate reproducibility. An unusual pattern in the data from the first phase suggested that there was a physical reason beyond operational concerns for the unacceptable variability. In the second phase, this new reason was investigated in an interlaboratory test involving six sites. It was determined that a physical phenomenon, termed shatter gap, was the principal cause of the irreproducible results from the first phase. For this setting, modifications of MIL-STD-662E to support reproducible results were suggested and successfully tested.