I sincerely believe the aforementioned firearms's portrayal is video games is at least partly to blame for the common misperception of them being especially apt at defeating rifle-resistant body armour, with one particular tittle especially coming to mind. This mythos isn't limited to PDW's however, but generally includes armour piercing cartridges for virtually all types of firearms. Particularly prominent examples are M995 and M993, for 5.56x45 and 7.62x51, respectively. These two cartridges are often seen as the be-all end-all of Western AP cartridges, and thus beliefs about their armour defeating capabilities are often greatly exaggerated.
I find the fact of the two aforementioned cartridges were designed and fielded not for a anti-personnel role, but rather an anti-materiel one, somewhat telling.