I've only encountered this with the M249 and the T88 LMG, and as zeroburrito said, it only happens when using red dot or ACOG, but not iron sights. It is also a bug and not a feature, which means the bullet drop is still there, however, the weapon hits above, where you actually aim. I wouldn't call that an advantage.
Ukratko, M249 i T88 LMG ''pucaju'' iznad nišana kada se koristi red dot i ACOG.