This is an American movie, violence is not an issue.
But show one nipple and you are in for a world of hurt.
I would agree that the concept of violence is not an issue, but there's a marked difference between how PG-13 and R
portray violence. PG-13 mostly alludes to violence or masks the full extent of the damage caused by a violent act. An R movie shows most everything in graphic detail.
An example would be: the PG-13 "The Dark Knight" alluded to the Joker cutting someone's cheeks. An R rated movie would've shown the Joker cutting that person's cheeks.
There is a lot of carnage towards the end of Mockingjay that will have to be swept under a quick cut, which is a bit disappointing considering the book took its time explaining the details of the carnage. I felt a little bit sick reading those parts.