8 inches of armor is something I would expect on a battleship except in their thickest belts.
I don’t think a .50 cal would even come close to penetrating that. Now maybe an RPG.
Did not find the video I saw but here is one of a 50bmg penetrating 2” steel plate.
http://www.metacafe.com/watch/yt—JP2FsqZS8E/steyr_hs_50_50bmg_3_sony_hdr_cx550v/
The other vid I saw was thicker, will look further for it. It has been a while.
Another issue is of course the bullet itself. That is where the magic is on penetrating armor.
Do you have one?