For those of you who want this to act like an actual .50 cal sniper rifle, that is to say 'One Shot One Kill' and not always to the head either (a .50 cal hitting any part of a human body will usually drive the person into shock if not killing them outright from blood loss), just mod these lines in the tweak file:
ObjectTemplate.minDamage 100
ObjectTemplate.damage 100
ObjectTemplate.distToStartLoseDamage 1200
ObjectTemplate.distToMinDamage 1600
Admittedly the ranges are far larger than the original BF2, but these came from my personal realism mod. Your ranges could be shorter. The point is that minimum damage is 100 so that gives the one shot kill.