Observed: Body armour and helmets are not very effective. Helmets can't stand any shots, not even pistol, and the best you can hope for the vest to do is to keep you alive if you get hit once but you still become wounded, and it's quite rare for the vest to actually do something at all to protect you.
Pistol shots: Don't penetrate vests. Don't penetrate helmets.
Assault rifles: Penetrate vests after 4-5 hits, until then causing no other damage than knocking you over. Penetrate helmets maybe half the time beyond 200-300m, if no penetration perhaps the wearer suffers from some groggy effect.
DMR: Marksmen rifles. Penetrate vests after 2-3 hits, knocking down until then. Penetrate helmets quite frequently, 3/4 times beyond 200-300m. If no penetration, stronger groggy effect.
Proper sniper rifles: Due to using rounds with more of a punch, instant penetration of helmet, and vests can stand a single hit beyond 500m.
.50 cal: Pentrate vests and helmets instantly.
Shotguns: No penetration from buckshot. No penetration from slugs, but strong groggy effect if helmet hit.