The M16 and M416 are great guns because they are extremely versatile. There's few engagements where you're at a disadvantage with those rifles, while most other guns in the game are specialty weapons. If you're using the AEK, you're taking away some of your longer range capabilities to boost your short range effectiveness. Snipers are the opposite. Shotguns eliminate all their long range effectiveness to dominate in CQC. With the M16 you can succeed in most situations, while with other weapons you have to control the engagement some. It's versatility makes it kinda OP.
https://www.youtube.com/watch?v=gGw-1p676-E