Personally the stats I would use for the assault rifle are 1d6+1 at a 16" range - exactly the same as what I use for Scythian Laser Carbines, you don't need to overpower everything for a game to be along the same lines as the halo mythos.
Hell, I used to run the FCOTRPGSOC - The Farnborough College Of Technology Role Play Game Society (try saying that three time when your drunk) Using the Halo RPG system I took from the net based off the fudge system - and what I learned very quickly is that NPC's that are overpowered, quickly become no fun to play with or against - as a result, I only featured one Scarab in the entire campaign - and that was a set piece for the characters to escape from before it was hit by a MAC blast from a destroyer in orbit.
Long story short - Don't OP everything doodski - keep your characters and abilities as close to brikwars standards as possible - and only give out certain abilities in certain areas. As for everything else - fudge it, fudge it to hell, its more fun that way.
