This is the f.o.b rifle, primary weapon of the Imperial Army and one of the earliest mainstays of the Black Age universe. The F.O.B. originated when I was in middle school, writing a juvenile science fiction short story that was basically set in the starcraft universe, but used different names for everything. The military units in the story used a laser rifle which I eventually named the F.O.B.
Well, since then, everything has changed except the name. This rifle is, in universe, the most current version of the F.O.B, which has been in service with the imperial army in one form or another for over 300 years. This one is the MBR-66, the last version, the MR-43, can be found here [link]
The MBR-66 is an updated version of the MR-43, the new ignition system, while slightly less powerful than the 43, is much more controllable and efficient, allowing the average soldier to carry more ammunition, the weapon itself is also lighter, being slightly shorter and implementing several "usability comforts" such as a adjustable stock and pistol grip, better ergonomics, recoil absorbers, stabilizers, accessory rails, and ambidextrous operation. However, true to the Imperial tradition of mixing old technology with new, the 66 retains several features that might seem out of date, such as a transportation safety and as overzealous heat dissipation system. Nevertheless, the 66 is effective as a battle rifle and its long effective range as well as its incredible reliability make it popular with the troops that use it as well as a tactical asset for the generals that send those troops into battle.