A friend once showed me his gun safe he had made from concrete and rebar. It was like an underground cellar outside the house with the entrance being from inside the house basement from what made to look like a standard closet. He stated that around the door perimeter, wall joining the house foundation, and safe ceiling, the concrete was 12" thick and reinforced with two layers of cross-hatched rebar on 4" centers. The other walls were constructed of similar design but were 6" thick. The safe ceiling was constructed about 2 1/2 to 3 feet below the outside grade and covered with dirt. The outside area above the safe was where he built a dog pen for his two Rottweilers. He then made a typical safe door from 3/8 plate and multiple keyed locks.