Answer

Im sure MSI gave a good answer, but I cant see it, so...
A railgun is a device that uses electromagnetic force to launch high velocity projectiles, by means of a sliding armature that is accelerated along a pair of conductive rails.
You can Google it to learn more.

Answer
Sorry for the double-post, but to give a very basic answer to the question: Its a bullet. It is just fired much faster than a normal gun, and therefore has the ability to pierce tanks.
Customer support service by UserEcho
Sorry for the double-post, but to give a very basic answer to the question: Its a bullet. It is just fired much faster than a normal gun, and therefore has the ability to pierce tanks.