Railguns use magnetic acceleration to propel metal slivers at very high velocity. They are accurate, reasonably silent, and signature-free - but can be detected by their electomagnetic signature. Because the darts are so light, they can have a great ammo capacity and a low ammo cost, but special amunition for needlers is prohibetively expensive.
Railguns are common among civilians on worlds where they are not prohibited. They are sometimes issued to police and militia forces, but rarely used by the military because of the need for supply.