What Exactly is a V50 Rating, Anyway?

During the course of testing on any piece of body armor under consideration for certification, the NIJ (National Institute of Justice) measures the speed of every round fired at the ballistic target, whether that's an aramid panel, hard armor plate, or a helmet. To qualify for the NIJ's tough standards, every piece of armor must undergo this strict testing repeatedly and with intense attention to detail. Every data point is measured, from the firearm used, the grains of the projectile, the thickness of the material being tested, on down to temperature and atmospheric conditions observed on the day of testing.

At a specified distance, a number of rounds (usually 6) are fired into the armor one by one in such a way as to match as closely as possible with the conditions most likely to be present in real world scenarios. These tests determine what speed would be required for the round to pass through the material 50% of the time as accurately as possible, and they call that speed the V50 (for velocity-50%) rating.

Significant quality control regimes are used to find this information using exacting standards and performed at a very high level of specificity. The NIJ uses labs like National Technical Systems to perform the testing, who also must be thoroughly certified. The NIJ and the labs they use have more information regarding the standards and procedures that are used.