Military close combat with a helmet..
Hm: "Improvise, Adapt and Overcome!"
I know how to use a lot of different military gear for close combat, but I don't think I have tried the helmet as an offensive weapon..
But if you have nothing else you must improvise..
Bayonet or knife is an excellent close combat weapon if you know how to use it..
I think it's a great, and actually fun thing to train with.
But if you don't know how to use it properly it's very easy for someone with a little skill to disarm and disable you.