Laser cannons were an offensive, laser-based directed-energy weapon equipped primarily on Simulant Warships.
A virtual character once mentioned that SSS Esperanto had laser cannons ("Back to Reality"), but since this took place in virtual reality it is unclear if Space Corps vessels actually carried laser cannons. JMC mining spacecraft like Red Dwarf certainly didn't.
Before the crew of Red Dwarf obtained laser cannons, the Cat would sometimes suggest using them, only for Kryten to point out the flaw in his plan - they didn't have any - twice. Starbug has a nuclear missile launcher, used by Lister to play pool with planets (in "White Hole"), limpet mines ("Back to Reality") and Starbug also used nitroglycerin in compacted garbage bombs to neutralise incoming asteroids ("Psirens"). Obviously, neither method could be used in close-combat space battles.
A Simulant Captain once ordered his crew to upgrade the vastly inferior Starbug and outfit the unarmed shuttle with laser cannons, so the boys from the Dwarf might be able to fight back and prove some sport. It proved their undoing.
The laser cannons on Starbug were too small to be seen, but were still effective, and were located somewhere on the top of the mid-section of the 'bug. The Simulants expected the Starbuggers to turn and run, but they instead used the cannons to fire on the Simulant Battle Cruiser, getting in an unexpected lucky hit which crippled it and killed most of the Simulants. ("Gunmen of the Apocalypse", Series VI)
When Rimmer was marooned in an escape pod that was faster than Starbug, Kryten suggested that they slow the pod down with a burst from the laser cannons. Cat suggested that they form an orderly queue behind the gun sight. ("Rimmerworld")
When attacked by themselves from fifteen years into the future, the Starbuggers exchanged laser cannon fire with the future 'bug, crippling it. However, the present 'bug was still destroyed by the vastly superior future 'bug. Paradoxically, Rimmer destroying the Time Drive with a bazookoid at the last moment brought them back from the dead. ("Out of Time", "Tikka to Ride")
It is possible that the paradox-altered Starbug of Series VII does not have the laser cannons anymore, since they are not used, for example with the Kinitawowi battle cruiser or SS Centauri. However, the superior flying skills of newly acquired crewmember and veteran navigation officer Kristine Kochanski, are used to race and cause enemy ships to crash on planetary surfaces.