Seems to be a few issues here between the bullet types.
To my experience, yesterday, I played quite a few rounds in which I used the Mk18 and the Mk17.
Mk18 being 5.56, I had to spray the everliving hell out of people to make a kill, while the Mk17 only required 1-3 bullets tops to actually score a kill at all.
The damage variance between the bullet types is absolutely absurd as it stands and needs to be rebalanced. I shouldn't have to unload a full mag watching blood spray out of someone with a Mk18 to make a kill simply because it's 5.56, while the Mk17 requires 1-3 bullets. The difference shouldn't be that overly comical to kill someone.