Kat's shields were down probably because of the Radiation Spike generated from the Covenant Cruiser's Glassing Beam.

True true. But their helmets can't take a projectile?

Depends on the kind of projectile. Shrapnel from an explosion/grenade? A .22 bullet? Sure. Long, unconventional, explosive crystalline spikes developed by a Tier-2 Alien Race? Nope. And the other Spartans' helmets weren't bullet-proof. You can kill Spartans by headshot in Multiplayer. They just made Noble Team invulnerable during Gameplay because they are essential to the story and you can't program an AI to be as intelligent and skilled as a player.

Oh ok true. So those needles should have just broken right?