Chain rifle

The Chain Rifle was a weapon borne by Marines in the StarCraft beta, acting as the predecessor to the C-14 Gauss Rifle in a sense. However, while it is compatible with a bayonet, it appears to feature a drastically smaller clip. Interestingly, it bears structural resemblence to the C-141 AC Gauss Rifle.

Note that it is possible that the Chain Rifle is an alternate name for the Chain-gun Cannon. Also note that as it only appears in early builds of StarCraft, its canon status is dubious.