"V-50" or" Velocity-50%" - is a ballistic test where bullets are fired at higher and higher velocities until they start penetrating. The velocity of the bullets where 50% of the bullets DON'T penetrate, and 50% of the bullets DO penetrate is the V-50 rating for that ballistic protection.

So, V-50 is a good measure of the UPPER limit of protection, or the "breaking point" for armor. You would want a V-50 velocity rating somewhat HIGHER than you would want for a Pass/Fail test (or a V-0 test = 0% of bullets penetrating).