Only precision weapons. A headshot with the AR, SR, SP, PP or BS (except for charged, which still doesn't care about shields/head) aren't one shot kills even if unshielded and headshot.. .
The Boltshot is headshot capable
Good point. I didn't know that until after reading this and then checking it out. Looks confirmed. So BS can also do a headshot, but it takes a full magazine, no charge shot, to take down a shield. Meaning you'll have to use the entire mag to drop the shield, then get the headshot on your 11th. Not so bueno. Especially since if you're in melee range, a charged shot is just easier OSK with it.
But, yeah, you're right which does increase its usefulness (slightly) in team games, much like the Carbine is useful as an assist machine.