Airsoft is a primarily recreational activity where replica firearms shooting plastic rounds are used for personal collection, gaming (similar to Paintball), or professional training purposes (Military Simulations and Police training exercises). A primary difference between airsoft firearms and BB guns is that airsoft uses 6mm or 8mm plastic pellets and have muzzle velocities of less than 700 feet per second (FPS), which is generally considered safe when used in a controlled environment, while BB guns utilize metal ammunition and shoot at higher velocities (up to and over 1000 FPS), making them unsuitable for gaming and training purposes. Another distinguishing factor between BB guns and airsoft guns is that the latter generally uses hop-up (back-spin on the BB) to increase their effective range, since the lower muzzle velocity of these guns shortens their shooting distance.