MA5 series
From Halopedia, the Halo wiki
Template:Weapon Template:FactOrFiction
The MA5 Assault Rifle has been in use since before the Human-Covenant War. It appears to be the predecessor to the MA5B-C series. The name itself could be the shortened version of MA5B.
But most likely, this refers to the MA5 firearm series (MA5B, MA5C and MA5K).