My friend has a 50 yard tunnel made of 8' culvert that was insulated and waterproofed. At the far end is the exhaust fan and filter system, the back wall is concrete protected by 2' thick layers of sandbags. In front of that is a hardened steel plate set at 45 degrees to deflect bullets down into a 8" bed of sand. The culvert has drainage system, concrete poured for a walkway and a target retrieval system.
It went on the house plans as a storm shelter and wine cellar. There is a positive pressure AC system blowing all air down the tunnel for safety