There is a skill upgrade that increases chances of Critical Hits, but what are critical hits in the game? Do they provide one shot kills? How useful are they against the NPCs?
Critical Hits have same meaning as in previous games in RE series, and are powerful attacks that do more damage to enemy and, for some enemies, causes different animation of their death.
Note that for most weapons/enemies causing a critical hit requires hitting enemy in special spots of their body, so that will not apply to your every shot (as general gun power upgrade). They help you to conserve ammo with small enemy encounters then you have time for precise shots, but less valuable then confronting large groups of enemies. That's up to you to decide that suits you better.