A couple of threads ago someone complained that CE pikemen kept headshoting his colonists, leaving them brain damaged, so I decided to do a little testing.
First, pikeman uses needle gun with massive range and fires this sabot ammo with 10 damage, 21 sharp and 73 blunt penetration.
If we compare it to some common head armor - for example masterwork plasteel helmet has 12 sharp protection, 21>12, so bullet will go through and deal roughly a bit less than half(about 4 or 5, it is percentage based on the remaining sharp penetration of the bullet) of it damage. Also, simple helmet only protects head and ears, leaving eyes, jaw and nose uncovered.
Masterwork marine helmet, on the other hand, covers everything and has 24 sharp protection(19.20 for eyes and nose specifically) 24>21, so bullet will not go through and will not cause sharp damage, bleeding and brain damage.
That being said, it will not protect you completely and forever, because bullets have blunt penetration too. Pikeman sabot rounds have 73 blunt, while marine helmet has only 53 blunt protection, in the result being shot in the head will still leave small bruises, witch can accumulate and lead to incapitating.