In the early 1970's I spent a lot of time shooting up cars to check penetration. If the windows were up and you were shooting at right angles to the door, a lot of rounds would enter the car interior. We did also use 3/4 plywood targets to see if there was enough energy to inflict a serious wound, but I don't recall those results.

The real issue was if there was any angle to the shot, virtually nothing went through, if it did, the deflections were weird and unpredictable. Even with .44 Magnum Kieth loads that would enter the trunk and leave through the grill on a straight on shot.

I recall an IPSC match in the 1980s where the Director used his new truck as "cover" for one of the stages. One of a group of cops from the local big city pulled a round low firing over the bed. We heard the impact and when the line was clear, we ran forward to look at the expected holes in the fender. No holes. The issue 158 RNL round impacted the inside lip of the bed where the outer & inner fender panels are spot welded together and only dented the metal. Didn't even knock the paint off.

The stories above about mail boxes are concerning. They used to be touted as cover.