Lasers use a Gennium-Arsenic laser crystal mounted to a power supply, usually an armored combat suit or external power pack. The GA crystal focuses the energy into a short burst of deadly firepower.
Lasers as firearms have been available for centuries, but are still very expensive and consume a great deal of power. On the other hand, there is no recoil when firing these weapons, making them a classic space Marine sidearm.
Laser pistols and rifles are equipped for double duty as painting lasers. They have comm-units and can relay shield information automatically when set to paint. (They cannot fire normally while painting.)
Additionally, military issue laser weapons come with couplings which allow them to integrate into the targeting and night vision systems in the helmets of all combat suits. Because of this integration, if these weapons are fired without being hooking into a combat helmet or targeting goggles, accuracy suffers.
Laser crystals can only endure short bursts of fire or they over-heat and crack, becoming worthless. Therefore, all standard issue laser firearms have governors which only allow single shots (or the rapid bursts of automatic fire). The average crystal can withstand over 3000 shots before it is so weakened that it must be replaced as part of general maintenance.