Yeah, your main issue there is the pistol. If you want one headshot to actually kill an enemy you need a decent amount of damage--and note that's damage per SHOT you're looking for, not damage per second. IIRC the basic pistol is listed as 100DPS, but its fairly fast rate of fire and quick reload means the actual damager per *shot* is only 20--that isn't going to one-hit anything even on a stealth crit.
Get yourself something like a hunting rifle, whose damage per shot is 66 and with generally higher crit damage, and it's totally possible to one-hit an enemy with a headshot.