Would the Ammunition Fired by an AR-15 Penetrate a Typical School Classroom Door with Lethal Force Remaining?

Title pretty much says it all. I know there are different kinds of ammo, and different kinds of doors. But most schools I’m familiar with have either a steel door, usually pretty heavy, or that classic birch veneer over a wooden core.

So for various common forms of ammunition, would a bullet fired into one of those doors likely come through, and if so, how deadly would it be?

Trying to keep this FQ territory. In case you’re wondering, there was something about officers being outside one of the rooms with the gunman and retreating. I’m wondering how dangerous it is on the other side of the door, assuming the door stays closed.

For this guy, 6 inches of wood. He doesn’t specify range and there was nothing to indicate remaining force on exit.

The door will offer zero protection.

Yes, a 5.56 x 45 mm NATO will penetrate the approximately 2 inches of a heavy wood door:

‘Steel’ exterior doors are typically a thin gauge mild steel over a wooden or fiberglass core. You can actually stick an axe through one with enough force. There are ‘bullet-resistant’ steel doors that have a high density composite but you won’t find these in schools because they are quite expensive. A solid steel door would weight far too much to be easily opened and closed and would prevent easy egress in an emergency.

ETA: Ninja’d on both options. I have to type faster.


And a conventional interior wall will not be much superior as a barrier at this range, save if the round hits a rebar or stud at an angle such that it is deflected or shatters, but it would still be a problem in whatever direction it winds up going.

Heavy doors are also a physical hazard for tiny, clumsy people.

As a general rule of thumb, a high-speed impactor will keep on going until it’s penetrated a cylinder with a mass equal to its own. Lead has a density of about ten times that of wood or flesh, so the total thickness of wood+flesh it penetrates will be about ten times the length of the bullet. Subtract off the thickness of the door, and the remaining penetration into flesh is much more than you’d like.

Materials with high tensile and shear strength can mitigate this, by effectively increasing the cross-sectional area of the penetrator, but wood isn’t too good for that.

Depends on what you mean by “conventional interior wall”. A lot of schools have cinder-block walls in the interior, and then often another layer of lockers and their contents on one side, and white/chalk boards on the other. That’ll do a lot more to stop a round than the wooden door will.

Good point there. If they have heavy cinderblock construction that will take a lot out of the projectile.

While this is largely true, a bullet can also be deflected by far less than a door. Somebody immediately on the other side of the door is getting little protection. Further away from the door, it’s harder to be sure of a hit. An officer deciding whether to take a shot should be taking into consideration whether he might hit somebody other than the target, and a door in the way might complicate that.

ETA: I read the question backward. If you are being shot at, a door is little to no protection as everybody else said.

A bigger consideration than deflection by the door would be blocking line of sight. It doesn’t matter precisely what direction the bullet exits the door in, if the shot wasn’t aimed to begin with, and even a paper door could prevent a shooter from aiming at a specific target.

Just want to say thanks for the info. I would have expected the bullet to go through the door, but I suppose I’d have expected a greater reduction in lethality.

This isn’t really the way bullets work, and t he terminal ballistics of firearms is a complicated topic that does not lend itself to a simple reduction to first principles of physics because of the complex interactions and deformation of bullet and target. For isntance, expanding or ‘hollowpoint’ bullets have a cavity in the bullet that causes it to expand when it enters a hydraulic medium (such as a body) which causes it to rapidly slow and delivery energy to a wound channel larger than the undeformed diameter of the bullet; these tend to be very effective on unprotected targets but offer much reduced protection against armored targets or those behind a hard barrier such as glass.

Lead or lead-core bullets behave the most like you describe but they will often deform on contact with hard surface, in many cases tumbling (‘keyholing’) which also significantly retards and alters the trajectory. In the case of high velocity rifle rounds, they are often moving so quickly and have such a high sectional density that they can cut through a couple of inches of mild steel and still retain enough momentum and kinetic energy to be quite lethal. Hardened or steel-core bullets can cut through many inches of mild steel or even through hardened steel because the bullet pushes the target material to the side rather than forward, so it can actually punch through many times the mass of the bullet albeit often at the expense of stripping away the jacket and surrounding material.


What happens with FMJ bullets and metal doors is the jacket sticks to the metal and the lead core squirts right on through, spraying fragments willy-nilly on the other side. If you are far enough away from the door the lethality will be greatly lessened. Within 6 - 8 feet of the door, not so much. I’ve fired .30-06 FMJ through 3/4 inch steel plates with no problem.

ETA: You beat me this time!

“I’m your huckleberry.”


Don’t forget they typically have a window in the top half of them. Glass is going to offer less protection than the thicker wood at the bottom.

That’s why there are two terms.

Cover - offers protection from projectiles

Concealment - hides you from observation

A closed and locked door with no windows is concealment, but not necessarily cover.

Less protection, but it can still deflect a bullet dramatically. Snipers do not like shoooting through glass for that reason.

I just watched a video showing a 9mm bullet going through five raw eggs in extreme slow-mo. The eggs barely slow the bullet down (about 20 fps per egg), but you can see the bullet visibly starting to deflect out of the first egg, and by the time it got to the end of the row (less than a foot) it was off target by enough that it completely missed the last egg after going through the center of the first one.

So a single raw egg say, 10ft in front of a target could cause a bullet to deflect enough to miss the target completely vennthough it goes through it with almost no loss of energy. Imagine now shooting through a pane of glass.

An AR-15 bullet is much more powerful than a 9mm handgun bullet, but as rifle rounds go it’s actually underpowered and for hunting is not considered a big game round but more of a varmint/hog/small deer gun at best. Some states won’t even allow you to shoot deer with a .223.

For example, the .5.56 NATO round the AR-15 uses has a muzzle energy of about 1200 foot pounds. 7.62X51 NATO rounds as used in most other nation’s equivalent weapons generate over 2,700 ft-lbs from a heavier, slower bullet. These will also deflect less when shot through doors or windows.

The military didn’t choose the AR-15/M-16 because it was deadlier than the M-14 or other military rifles. They chose it because it was theoretically lighter, a soldier could carry more ammo, at short-medium distances it was quite effective, and in war wounding a soldier can be more effective than killing him, because a wounded soldier is still out of the fight but now other soldiers are occupied treating, carrying,and evacuating the wounded.

Light, high speed bullets like the .223 deflect easily, which means they can tumble and cause a lot of wound damage when they hit someone, but that same characteristic means they are worse than average for shooting through any kind of obstacle at a target.

Seems to me that if a gunman were to shoot through a door’s glass, the purpose isn’t to kill people inside (who would likely be hunkering under desks anyway,) but rather, to knock out the window so he can put his hand in to manually open the classroom door.

Or the shooter could just go outside the building and start spraying bullets through the classroom windows.

I’m fairly certain that last part is a myth. We have even discussed it before (12 years ago!) though without any solid conclusion. If you have a cite for that, maybe resurrect this thread: