The Stream Cannon is an Aian weapon.
A long coilgun is attached to an enormous generator. The generator heats up a block of iron, which is then melted and grabbed in the coilgun's magnetic fields. Through the barrel (usually the length of the ship) the stream of molten metal is accelerated, usually to around 85% light speed, causing immense damage.
A 400kg stream at 85% light speed has 7.7 gigatons on kinetic energy.
Due to its liquid nature, a higher proportion of energy is transferred to the target. However, some of the stream will penetrate straight through, potentially causing even more damage.
Technology: Aian Ringworld, Stardeath Howitzer, Stream Cannon, Lightflame Bomb