I have to admit right off the bat, that I have no experience whatsoever in dispatching Sentries, and I have not read any of the military manuals on the topic.
What strikes me as slightly amusing is that I actually considered the technique of stabbing your knife through the side of the neck, so the blade ends up behind the "pipes" in the unfortunate sentry's neck and then you slice away from your self.
This always seemed like a better procedure than the one they always show in the movies, where they simply attempt to slice into the cartilage of the neck from the front.
Here's another one. How about stabbing the poor sod in the ear. And ram your blade in to the hilt, while you cover his mouth and nose with your free hand. Probably shouldn't do this with a large knife, as you'll most likely stab your own arm.
Then again, buy a Hossom megabowie with a 14 inch blade and decapitate the guy. Even if he is wearing a helmet, his neck is most likely still exposed, and if you aim it right you should be able to cut his head clean off. You can do the same thing with a Mad Dog Saxon Sword or any other tactical short sword. Why use a knife if you can carry a sword? It's a hell of a lot faster in my opinion and you don't even have to touch him to do it
Ack, this is getting macabre...
As a side note, even if you sever someones neck completely there is still going to be quite a lot of gurgling and wind noise as the air in the guys lungs leaves his body. This may not be possible, but for least amount of noise, one should wait with disposing the sentry until he has just exhaled...Talk about timing. Not something I would like to do myself, to tell you the truth, but oddly appropriate when you look at the meaning of my chosen nick...
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Regards
Joshua "Kage" Calvert
"Move like Water, strike like Thunder..."