We took one down in the late 90's with about 1500 rounds (if we would of actually been aiming I am sure would of been a whole lot less) of 7.62 and 2 Kalashnikov's. Got a video of it buried somewhere.
Tried 5.56 but just could not get the penetration thru both sides. The 7.62 would punch thru one side and the other. Funnest part was shooting the bands off first all the way up. Thought for sure that would of been enough for it to tumble but nope, we had to punch out most of the bottom.