Measuring the ballistic performance of armor is based on determining the kinetic energy of a bullet at impact (*E*_{k} = ½ *mv*^{2}). Because the energy of a bullet is a key factor in its penetrating capacity, velocity is used as the primary independent variable in ballistic testing. For most users the key measurement is the velocity at which no bullets will penetrate the armor. Measuring this zero penetration velocity (*v*_{0}) must take into account variability in armor performance and test variability. Ballistic testing has a number of sources of variability: the armor, test backing materials, bullet, casing, powder, primer and the gun barrel, to name a few.

Variability reduces the predictive power of a determination of V0. If for example, the *v*_{0} of an armor design is measured to be 1,600 ft/s (490 m/s) with a 9 mm FMJ bullet based on 30 shots, the test is only an estimate of the real *v*_{0} of this armor. The problem is variability. If the *v*_{0} is tested again with a second group of 30 shots on the same vest design, the result will not be identical.

Only a single low velocity penetrating shot is required to reduce the *v*_{0} value. The more shots made the lower the *v*_{0} will go. In terms of statistics, the zero penetration velocity is the tail end of the distribution curve. If the variability is known and the standard deviation can be calculated, one can rigorously set the V0 at a confidence interval. Test Standards now define how many shots must be used to estimate a *v*_{0} for the armor certification. This procedure defines a confidence interval of an estimate of *v*_{0}. (See “NIJ and HOSDB test methods”.)

*v*_{0} is difficult to measure, so a second concept has been developed in ballistic testing called the ballistic limit (*v*_{50}). This is the velocity at which 50 percent of the shots go through and 50 percent are stopped by the armor. US military standard MIL-STD-662F V50 Ballistic Test define a commonly used procedure for this measurement. The goal is to get three shots that penetrate that are slower than a second faster group of three shots that are stopped by the armor. These three high stops and three low penetrations can then be used to calculate a *v*_{50} velocity.

In practice this measurement of *v*_{50} requires 1–2 vest panels and 10–20 shots. A very useful concept in armor testing is the offset velocity between the *v*_{0} and *v*_{50}. If this offset has been measured for an armor design, then *v*_{50} data can be used to measure and estimate changes in *v*_{0}. For vest manufacturing, field evaluation and life testing both *v*_{0} and *v*_{50} are used. However, as a result of the simplicity of making *v*_{50} measurements, this method is more important for control of armor after certification.