The DCM-8 does in FACT have a higher fire rate than the Reaver. It's even in the stats if you don't believe me. (Perhaps not majorly noticeable to you, but still faster none the less) The reason the Reaver outguns the DCM-8 is because the Reaver has better range and damage. And I know, as I not only used the Reaver during the beginning of the game's life span, but I STILL use it to this day.
(Also, the DCM-8 is automatic. So all you have to do with that thing is just hold down the trigger. There is a reason the DCM-8 has more rounds in it's mag than the Reaver. The developers did it for game balancing purposes as it has a higher fire rate and takes more shots to kill someone with than the Reaver. )