Were you using the Foster style rifled slug? If so, I'm not surprised that it wouldn't penetrate. The solid (Brenneke) style probably would have penetrated the vest. See the data here: http://www.thefirearmsforum.com/showthread.php?t=121485Back in '06 when I went thru the firearms instructor class, we tried some Hornaday SXT 12 gauge slugs out of a riot gun with rifle sights. They were about as accurate as regular rifled slugs. The big difference came when we tried both against a level II (used) Balistic Vest. The rifled slug stayed in the front face of the vest. The Hornaday went thru both sides of the balistic vest just like it wasn't there.