I’m not that far along to have super great helmets. But you can see their stats when you pick them. I would assume there is some sort of logic to it. Sniper guns have different force and helmets have different resistance. But I don’t know for sure how it works in game.

are you not not winning?

Helmets do stop a bullet, but they fall of your head when doing so. Note that light armor + big sniperrifle hit can still mean 1 hit KO from full health.