Ahhh, nah man, i wouldn't call something that could take my head off from a mile away "stupid".
About the scenario... Hmmm, that really is a tough one, because i'm assuming his hand is on his gun. Well, in that case, and if his eyes/back of his neck etc tec are covered, and for arguments' sake he's bigger than i am then there is nothing i could do unless i was certain that my knife could penetrate his clothes. I'd avoid him if at all possible.
I'm not sure what the muffler around the neck does, but if his mouth were uncovered (which it should be so he could yell out or use the radio) a knife (if long enough - which it should be) can always go up thru the bottom of the jaw, thru the upper soft palate of the mouth, thru the nasal passage and up into the brain. But you'd still need to be careful of that weapon being fired and a warning going out.
I really don't know man, i've got no military training - just 18 years martial arts training. That counts for about, oh, i dunno, maybe an equivalent 2 or 3 days military training. So essentially i know next to nothing.
Rich.