LowObservable
The original 4,700 pound GBU-28s had bodies made from old 8-inch naval gun barrels, I believe, although eventually they ran out and went to custom cases.
//Nearly typed that as "gin barrels"
There was a programme on them, they were actually developed as an in house project on the side by a team at some ordinance company, when the military were looking at taking out some of Baghdad's bunkers they rushed some into production, the barrel was a cheap quick fix to the casing problems, they dropped one in the desert to see if it worked but had no way to see if it achieved a 200ft penetration. After the first drop the other one or two built were rushed to the Gulf and dropped, no one knew if it worked until ENDEX when they were able to see they had, the charge was really small, but in the enclosed bunker complex it was devastating.