Ballistics

Question
During a training exercise,(law enforcement) we fired a 40 sig sauer at a window. The top window was slid down behind the front window.(double pane X 2) The round penetrated 4 panes of glass total. Heres the question.... First penetrated pane had a bullet sized hole,the second had a larger hole, the third had a larger hole than the second. The fourth pane of glass had a hole the same size as the SECOND pane. We would like to know why this occured. Sounds like a riddle but it is a serious question. Can you explain or refer me to someone who can? Thanks in advance.

Answer
Leo,

This is a pretty interesting question and based on the research I did, I'm not sure there is a definitive answer out there for you.  There is just not a lot of information in forensic science on the behavior of bullets penetrating multiple layers of glass.  There is a lot of stuff on penetrating a single layer, just not multiples.

Maybe a short discussion of what happens when a bullet penetrates glass would be in order.  Essentially, when a bullet hits glass it starts to lose some of its energy.  It is also important to understand that glass has a certain level of elasticity.  It actually begins to flex on impact and at the point where the elastic coefficient is exceeded, the glass cracks and deforms.  This is why the direction of a bullet through a pane of glass can be determined.  The bullet hole forms a crater, with radial and concentric cracks surround the point of penetration.

As the bullet penetrates each successive layer of glass, a couple of things happen, first the bullet will start to lose velocity, second, if the bullet was jacketed, the jacket will separate and become an additional 'projectile'.  Additionally, shards of glass from the windows also become secondary projectiles.

My educated guess (and that is exactly what it is since I can really see the size or configuration of the holes, don't know the range that you fired from, the type of glass in the window, whether the round was JHP or Ball etc.) is that the first hole was the size of the unexpanded, undeformed round.  The second was larger due to round expansion/deformation. The third was larger still since there is now some accumulated debris from panes 1 and 2, plus the jacket could have separated and made the third hole a little larger.  

By now the bullet is losing some energy, the pressure wave is lessening significantly and the hole in pane 4 is probably about the size of the expanded round that penetrated pane 3.  The accumulated debris that made the hole in pane 3 larger probably ended up between panes 3 and 4 at the base of the window frame.  I'd bet that you might have seen pieces of the jacket there if it was jacketed hollow point ammo that you were shooting.

I am not a ballistics expert by any means, but the above explanation makes some logical sense.  

I am honestly not aware of anyone doing this type of ballistic research.  Most of the folks out there are doing stuff that relates to the effects of particular types of ammo in tissue and the effectiveness of a particular type of round in a type of game (deer, javalina, elk, bear, etc.)

There were a couple of FBI and Forensic Science articles that I found on glass penetration, but they all focused on energy loss of the round and not on any other aspect of the problem.

I can tell you that I will be trying this on the range myself sometime this summer.  You have my curiosity aroused now...

Hope this give you some food for thought.

Doug