the fn 5.7x28mm is a pistol-sized, armor-piercing bullet made by the belgian gun company fn herstal. fn herstal made the bullet because nato wanted a pistol-sized round that could replace the 9x19mm parabellum bullet, since the 9mm cannot shoot through most body armor. however, the fn 5.7x28mm never completely replaced the 9mm because germany vetoed the decision. it has a similar design to the 5.56x45mm nato because it is small, but has a lot of stopping power, accuracy, and penetration because of its high velocity. guns that use the fn 5.7x28mm include the fn p90 and the fn five-seven.