Naruyashan
Reality's Janitor
I might just be misremembering it, but in one of the novels there was a magnum that was specifically said to fire higher caliber rounds (or something to that effect), and they were also said to have an advanced targeting system. I believe it was in the novels that covered the mission to save Halsey on Reach. Like I said, I might be wrong, I'm just going off of what I remember.