One shot kill snipers with 3 plates?

I assume I'm not building my snipers right or something. Because I've gotten one-shotted through 3 plates by the AMR, and 7.62 (I'm aware some of the MW3 snipers can one shot) and even if I hit a headshot with the Sniper, it's never a kill through full plates. Do I need to equip something on my guns or what's the deal?

Random Edit because I thought of it. I have been using long barrel and the over pressured rounds