1. Sheetrock (drywall) doesn't slow any round down much. If you shoot in the house, walls will not stop any serious round.
2. Twelve pine boards will not stop a .223 round.
3. Shooting stuff is fun.
(My favorite, in the section on whether thick clothing provides protection against bullets: "If the North Koreans seemed uneffected by the .30 Carbine rounds, they were probably not being hit. Because clothing won't stop a 30 Carbine.")