It may not stop the bullet, but it certainly will slow it down...
My firsthand witnessing of this subject is as follows:
1) One night at the local 7/11, this guy zooms in and parks and gets out and walks around his car saying "bad words." I noticed on the back of his car were two or three rather large dimples that were bare metal (paint gone).
This agitated guy said someone on the freeway just fired shots at him. I don't know what the caliber was, but if I had to guess based on the appearance of the dimples, I would guess .380 or 9mm.
2) Late one night I took my dad into the emergency room for an urgent matter. A young gentleman was standing patiently in line with the rest of us waiting his turn for the triage nurse. When his turn came up, She said "Can I help you?" to which he responded, " I was just in my car when someone walked up to me and shot me in the face." They rushed him into the back with doctors and nurses frantically converging at his bedside while they awaited an ambulance(the hospital was not a trauma facility.) Of course the question I asked myself is how does someone get shot in the face, drive to the hospital, and then stand in line? Having some time to kill waiting for my dad, I went out to the parking lot and sure enough I found a car with a bullet hole in the driver's side windshield.The hole was approx. 3/8" diameter. After a few minutes of thinking about it, I concluded that the angle of the windshield and likely the thick plastic membrane sandwiched between the two panes of glass (for those of you that don't know this, a windshield consists of two thin panes of glass with a tough plastic membrane sandwiched in between them),took enough speed off the bullet that it was stopped short somewhere in his sinus cavity thus it did not enter his brain. Based on these two examples, I believe a door would provide good cover against 9mm and smaller rounds and slow down or stop more powerfull rounds. Anyone that works on their car such as myself knows that inside the door are some fairly dense metal parts and with a small amount of luck even a fairly powerfull handgun round hitting some of these parts, would be stopped.
