Well, normal civilian have 60~150 hit points, so basically what you need to do is made sure it deal ~100dmg at 5m mark. Since JE have mention that the explosive decade is by index of 2, i.e. Base DMG^1/target radius(by in game unit, which seems to be inches/cm)=damage.
Eitherway, weapons in NV are way less powerful than their real world counter part; i.e. a 9mm in the head should reduce your have to near zero without any sort helmet, fire that burns your skin would cause all sort of shock and pain to prevent you to do anything, or 50cal would blow a big hole in whatever living thing that it hits.
I agree with you in theory. Since it is completely within the realm of possabilities that a single .22cal round would kill an unprotected human, then that should be possible. Now, it takes a fair ammount of skill/luck for this to happen, so that's where the RPG mechanics come in. But you should be able to kill any human target, given the most ideal circumstances, ie: maxed skill, maxed weaponnhealth, most damaging ammo, most advantageous circumstances (stealth critical). You then work your way backwards to base damage from that.
Completely hypothetical situation:
Assume the highest level human target, and the body part with the least hit points has 100 hit points.
Assume stealth crit dam of 200% of base damage.
Assume ammo dam miltiplier of 1.75%.
The base weapon damage (with max skill and max weapon health) would have to be 19 (if I did the math right) to kill a 100 hit point target under the most advantageous circumstances. Factor in damage midifying perks and it could be lower.
Since frag 'nades have no crits, they would have to deal a base 100 dam to kill the above target.
That brings up the biggest question of them all:
Under what circumstances should any single hit scored on an enemy result in damage enough to immediately kill that target?-Gunny out.