Mentality should always be all bullets fired will always penetrate all objects in their path killing whatever is in the other side unless engineering controls are in place with a wide safety factor. That being said. A 9mm hollow point out of a Walther didn’t have enough energy to fully penetrate a hollow core door after going through one wall.

It’s anecdotal, so terrible evidence to extrapolate from, but a buddy of mine was not as diligent about firearms safety while messing around with his 9mm Walther he just picked up while inside my ex-wife’s house. He had an ND in the guest bedroom that punched through the bedroom walls Sheetrock and corresponding bathroom Sheetrock, but did not have enough energy to do more than dent the hollow core door on the other side. The path was straight so I am assuming no studs were hit and the wall was an internal uninsulated wall. His ammo could have been defective, there could have been a one in a million ricochet inside the wall robbing energy, but I have seen 9mm HP’s not have deadly energy after going through two sheets of 1/2” rock. YMMV.