Plasma weaponry

The Plasma Weaponry the Covenant employ always follow a distinct pattern regardless of the specific gun;it it is stabilized using magnetic fields. Without the field, the plasma dissipates too quickly to be of any real use. The field controls and contains the plasma, without interfering with firing. The plasma sword is an example of this technology as the magnetic field holds the blade together while allowing it to cut through objects. This is also how the plasma pistol's overcharge shot works. A small magnetic generator is installed in the hilt of the pistol and, when fired, the bolt tracks its target using a very simple motion tracking system built into the magnetic generator which alters the field to make the plasma move. A larger version of this effect is seen in the plasma turrets of Covenant cruisers.

This information was obtained by Cortana, a UNSC AI, while she was in direct contact with a Covenant ship's data core. It is reasonable to assume this information is accurate. (From Halo: First Strike)