Do helmets stop bullets?

[Harrifer]

So, we all know when the infantry get they helmets popped off by enemy bullets, but would that bullet have otherwise killed them?

Or is it that the helmet is bigger than their head and so more likely to get hit?

I ask because i always make them go back and get their helmet, just in case. The helmet also takes up room in the inventory, so i figure it must be there for a reason.

Also, do men run slower with more equipment? I haven't raced them yet to find out.