How much velocity does a bullet have AFTER going through your wall?

     I am really excited to be publishing this post because it ties together so many things I enjoy.  Shooting, math, photography, and tangible useful results!  The idea came to me after reading another blog (I feel terrible but I can't remember the name) about how many walls various rounds would penetrate.  They lined up sets of miniaturized drywall sections one after the other and concluded at the end that X caliber with X bullet type would punch through X number of "walls".  Now this is interesting don't get me wrong but it just didn't do it for me.  I wanted to know exactly how it affected the bullet once it flew through into the other room if you will.



     To find out I decided to perform some DIY carpentry of my own and built a couple 8" x 8" simulated walls using two sheets of 1/2" standard drywall and 2x4's I had sitting in the garage.  The plan was to place these directly in front of my chronograph, a device that measures an objects velocity by timing how long it takes for the object to cross two skyward facing lenses.

   Please don't shoot my chronograph, please don't shoot my chronograph

I've jinxed myself, I am totally going to shoot my chronograph...

       In order to make this test as accurate as possible I shot five times without the drywall to get a good average muzzle velocity without any barriers.  After I had that number I shot into the "wall" and once the bullet came out the back there was my chronograph waiting to calculate the speed.  I did this three times with each gun/ammo combo to get another average velocity figure.  I wanted to make sure there wasn't erratic behavior post drywall, for example wide swings in speed or something.  I only conducted single wall tests as this would give me a worst case scenario and honestly if you start flinging bullets through one wall it is already a worst case scenario isn't it?  I used a full sized Glock 17, compact Kahr PM9(both 9mm), and an M4 rifle in 223 with 16" barrel, all common self defense guns.  In the two pistols I used the very popular Federal 124gr +P HST and in the M4 I shot hand loaded 55gr Vmax bullets.  Ok enough about the setup, let's get to the results.

The chronograph survives!

    

      A graph showing the feet per second velocity of the rounds tested without the barrier and the remaining velocity after the barrier:

     Holy crap!  I knew the rounds wouldn't be slowed down by 75% or anything but wow, Only the Kahr had more than a 5% cut in velocity.  The Glock 17 with HST bullet was only reduced by 1.85%, the Kahr with the same rounds was reduced by 10.8%, and the 223 out of the M4 rifle was reduced a minimal 4.2%.  No wonder bullets routinely slice through multiple walls in tests, they aren't hardly affected.  This really drives home the point that you have to train and train and then train some more.  You just can't afford to miss.  Speaking of which...

     What happens when you actually hit your target but the bullet passes through?  I setup my last remaining wall segment with two gallon jugs in front of it and one behind it.  Disclaimer:  Water jugs do not replicate a human body and you cannot rely on this test alone.  I am merely showing what happens when a bullet passes through a water dense material, expands, and then hits the drywall.  I used the Glock 17 (Glockianna) and the HST round again.

Threat!!!!

These jugs can't hurt you anymore....

     End result?  The bullet passed through the two jugs in front, dousing me with water in the process, and passed through one sheet of drywall but not the other!  I found it fully expanded in between the two sheets face up on the table.  It dented the other side but that was it.  Moral of the story?  Use good quality ammo and hit the bad guy first instead of your walls.  Water logged bodies do a better job of slowing down a round than gypsum and air.  In case you were wondering, the bullet retained 100% of its weight and expanded to 0.749".  

 Super realistic home defense dramatization photo, may not be suitable for all audiences. 

I seeeee you.