I had an uncle in 'Nam. He says they used crossbows frequently, and it wasn't uncommon to see a patrol out there with one. They were used mainly for "silent interdiction" of sentries or guards when either sweeping a base of operations or prisoner extraction. Alot of SF guys had them. One thing he heard from a guy back then was something along the lines of "In the bush you can make bolts from chickenfeathers and saplings. Can't make bullets."
Just thought I'd share