From what I can gather, being undetected multiplies your weapon damage to some degree (likely 2x). Combine that with a lot of low HP mobs walking around can lead to the illusion that being in stealth automatically makes your shots instakills.
Tier 1 level, using p416 which has not upg aded will not kill a Unided soilder with one shot on the chest.
When you use the G28 scope use also the range meter.
