idk but something safe probably.
Unless you're an engineer or otherwise technically-inclined builder comforming to published NRA range safety standards, "safe basement range" is an oxymoron.
he said it shook his whole house when he shot it.
A bit of advice: ranges aren't supposed to shake.
he only shot it 3 times to break in the barrel.
Now this point is debatable, but many experts who recommend "breaking in" a rifle barrel call for several more than just three rounds. One round, clean with strong solvent, oneround, clean with strong solvent, and on and on and on....
Like I said, Marines are the supermen of the firearms world. They're just damned fine combat personnel.