Topic: Windshield perforation angles

[Brandon Bertolli]

Can you determine the angle at which a windshield was perforated if you don't have any evidence on either side of the glass? What I mean is, if I perforate a windshield and bring you just the windshield can you tell me what the angle of incidence of bullet to glass was?

I understand that the bullet changes trajectory upon impact because of the slope of the glass relative to the nose of the projectile. I've been told that a horizontally-fired bullet that hits the exterior of a sloped windshield will have a minor trajectory change towards the ground and that a horizontally-fired bullet that hits the interior of a windshield will have a trajectory change towards the sky.

So my second question is:

Is there enough thickness of windshield glass and adequate techniques available to determine what the trajectory change was, in circumstances where the round is known to be intact, but the round has not been recovered and nothing else about the shooting is known?

Thanks again for the knowledge.

[Geoff Bruton]

Good morning, Brandon,

This is an interesting question.  Most of the work involved in the calculations for angle of incidence depend upon the nature of the substrate being hard and unyielding.  Although empirical testing is always beneficial, the calculations in this regard appear to be analogous to those for bloodstain pattern analysis.

However, upon closer inspection of what is actually occurring during bullet-windshield interaction, I think that one must be very cautious when attempting to estimate the vertical angle element.  Obviously, when compared to bloodstain pattern interpretation, the blood is yielding whilst the substrate is typically not.  Conversely, whilst the bullet is relatively hard, it may deform and/or fragment, not to mention be deflected to some degree (as you already mentioned).  In addition, the windshield itself is not 'unyielding' - the laminate flexs quite dramatically during impact and oscillates, and can also break irregularly from shot to shot.

On page 180 of Ed Hueske's book, "Practical Analysis and Reconstruction of Shooting Incidents" (part of the Practical Aspects of Criminal and Forensic Investigations Series), Ed writes:

"The vertical angle cannot be reliably determined from the length and width measurements of bullet holes through windshields because of bullet deformation and fragmentation and the irregular breakage of windshield glass.  In the one test conducted by the author with 40 caliber Federal Tactical rounds, no reproducible results could be obtained and none of the vertical angles of impact calculated from bullet hole width-to-length ratios were within fewer than 15 degrees of the actual."

Personally, I am always in favor of actually doing some testing of my own - and this sounds like a great day at the range!  If you know the firearm and ammunition believed to be responsible for the shooting incident, I would go ahead and do some testing - but being mindful of the potential limitations.  If you do decide to go this route, please be sure to share your findings!

Best wishes and warm regards to all,
-G.

[Brandon Bertolli]

Geoff, thanks for such a detailed answer. It looks like there is no easy way to determine this, then. At least not with the glass alone.

[Michael Haag]

Hey Brandon.
Agree with Geoff.  Empirical testing is always best.
Thought I'd throw in 2 cents here also.
For the azimuth angle, the long axis of the ellipse is good, but only as viewed from directly above the impact, with some relevant reference line in view.
I put little to no resolving power in the elliptical shape of the hole for the vertical component (for common angles of impact), however.  As Geoff said, because glass is so frangible, the proportionality of small d to big D ellipse is not consistently representative to any degree of certainty other than saying it is a relatively flat trajectory (assuming we are dealing with the common approach angles we usually see for windshields).  Because of this lack of resolving power, the vert component usually doesn't help to position a firearm 5 vs 50 ft away.
As for the deflection issue, I have done several well documented experiments, as well as multiple less intense studies at my shooting recon classes involving deflection, and for handgun bullets going into the glass at the common windshield angles, downward deflection on the order of 1 to 10 degrees is common.  This translates to about 1 to 6 inches downward at the common range of a shootee from the windshield.  Slight right to left seems unpredictable.  Shoot outward, and indeed the deflection changes direction.  Important to note here that the distance from the windshield to target may now increase so observed deflection can be significantly greater.  However, be cautious in that once you get extremely shallow, or change velocities, the deflections can be different...  This general rule applies to common handgun bullets and velocities.
The "bloodstain" trig functions do give good approximations of bullet impact angle in materials like drywall where the front half of the perforation is fairly intact...  I have done known angle verifications of this...  a minimum of +- 5 degrees seems appropriate, like with many other trajectories.
Sorry to be so long winded.
mh
   

[Brandon Bertolli]

Thanks, Michael, that is great information. I do appreciate everybody's effort here in educating me on this issue.

I am going to have to read up on blood stain trig functions because I am ignorant in that regard. But generally, from what I hear so far, it seems to me that even if you had a windshield perforation and a fabric breach in a car seat, you couldn't work out the angle of incidence of the bullet to the glass, because of the unknown deviation in trajectory from the time the bullet interacted with the glass to the time it hit the seat or whatever target it hit within the vehicle. Is that a fair statement to make, or can you specify a range of angles if you know roughly what the velocity of the projectile was and you recovered it intact?

[Michael Haag]

From my experience, I would suggest a minimum of +- 5 degrees if you have a subsequent impact in the seat.  Depending on how shallow your impact is to the glass, maybe even +- 10 degrees.  Remember the tendency with handgun bullets at common velocities will give slight downward deflection (out to in)...
The azimuth component is usually more informative becasue there are a wider range of realistic possibilities in the azimuth plane (ie, all around the car), whereas the vertical components (which may normally be used to establish gun location out from the vehicle)  of such shots are usually pretty "flat" / horizonatal.  The vert component's accuracy in glass isn't suffient to get accurate info becasue the +-5 or 10 degrees opens up the range of possibilities so wide.  This isn't just glass though, as establishing gun from object distances with flat trajectories is difficult for these same reasons...  The steeper the shot, the easier it is.
Tough concepts to type.  I hope that makes sense. 
Take Geoff's advise though...
Shoot stuff under controlled conditions in order to give yourself a proper education in these areas.

mgh

 

[Bob Kennington]

"...In addition, the windshield itself is not 'unyielding' - the laminate flexs quite dramatically during impact and oscillates, and can also break irregularly from shot to shot..."

Some readers may not know of US' windshield requirements.

European windshields  (last I checked) are not laminated—but tempered. (Laminated means having a plastic ply between two glass sufaces). Side windows and the rear window are tempered.

Also, not all windshields are alike: modern bonded windshields are much thinner and lighter than their predecessors. (And why we see so many windshields being penetrated—and passengers struck—by workaday construction debris tossed up from roadways today.)

I would shoot tests with the windshield bonded in place, as the windshield's strength is otherwise compromised.

Sometime, watch the reflection of a modern windshield outside while someone cleans the inside surface. You can actually see the glass flexing at each finger: scary and impressive at the same time.

[Brandon Bertolli]

@ All

That's interesting information regarding laminated vs tempered glass.
I have a case in my manuscript (not yet a published book) that involves a bullet that perforated the side window of a VW Golf and continued on a downward trajectory and struck the driver on the left thigh. The bullet remains in the patient, but the wounds and the radiological appearances of the bullet are helpful in verifying the patient's statement in terms of the circumstances of the shooting. I'll find one of the radiographs and post it here. The bullet has a definite flattened nose and I am wondering if the angle of deformity relative to the long axis of the bullet is a close enough match to the angle of glass relative to the barrel at the time of the shooting.

[Michael Haag]

Adding on to what Bob said, not all laminated windshield are the same in construction.
Some more expensive cars (BMW, Mercedes, Lexus, etc.) have multiple layers of laminate and glass, ie, more than just one sheet of plastic as is most common.  I have heard of but not observed 5x layers.
Unfortunately, I have never had a BMW shot in ABQ.  I'm still waiting.
Also, refer to "Shooting Incident Reconstruction" by my Padre, Luke, for info about the "flat" spot deformation vs. angle of impact. Particlularly in cases when rico / departure angle is not reflective of the incident angle (as in most cases), this damage is the only good way to estimate in-shoot / incident angle.  In short, yes, it is a good approximation of the angle of impact, assuming significant subsequesnt damage was not done to the projectile.  A nice soft leg should be a great decelerator as opposed to steel structural material.  This is also one of the many pratical demonstrations at shooting recon classes.
I'd be curious though how to insure with a radiograph you are getting a propper orientation view to assess this angle on the bullet still in a leg.
Look forward to the book!
mgh   

[Brandon Bertolli]

Ladies and gents, I have located the radiographs:

No 1: AP Pelvis



No 2: Horizontal beam R thigh



It was only a matter of good fortune that the standard radiographic planes resulted in radiographs being obtained roughly parallel and perpendicular to the long axis of the bullet. I don't think you could measure the angle within 1 degree accuracy but it is my claim that in this case the radiographs might be helpful in determining the angle of incidence.
Unfortunately, because of the density of lead, if you don't have the X-ray beam tangential to the impact surface of the bullet, you can come to misleading conclusions about the state of the bullet. You can emulate the beam projection dynamics by drilling a small hole through the length of a bullet and inserting a glass rod through it. You can then get an idea of the radiographic contour by shining an ordinary flashlight on the bullet and observing the shadow on a piece of card.
I guess you could also ray-trace it with 3D modelling software which gives you the advantage of being able to modify the geometry of the impact deformity on the fly.

Anyway, I fully appreciate the problems of documenting an impact surface radiologically, because even if you have the base of the core relatively flat on the radiograph, you are still left with variables associated with the rotation of the bullet about its long axis, relative to the beam. If this case went to court, I would probably suggest multiple projections, covering a 20 degree arc (10 degrees posterior and 10 degrees anterior angulation relative the the original horizontal beam lateral). If I get the time this week, I'll do some ray-traces to illustrate what I mean.

By the way, here are the images I have used to illustrate the possible trajectory of the bullet in this case. I didn't create the model of the Golf, but I did add some furnishings inside the vehicle because it was provided on-line as a mere shell. There are several small discrepancies based on the fact that this was not the exact model of the Golf in question and also the colour is not accurate. I was not allowed to see the vehicle, but the patient offered detailed information about the circumstances of the shooting.





I have wound photographs also, but I am holding those back because they are of a sensitive nature. The man's scrotum was perforated and the photographs include his penis. You'll have to trust me when I say the entrance wound was consistent with a round that had already expanded or was otherwise deformed prior to impact.

On the radiographs you can see the paperclips that we have used to mark the skin breaches in this case. The position of those paperclips radiologically can help prove what the position of the patient was, at the time of the shooting. I'll give you a hint: on the AP view the paperclips don't line up because the patient was X-rayed supine.

[Brandon Bertolli]

I must add also, that it is not clear who the target in this shooting was, because the shot was fired from the passenger side of the vehicle (which is right hand drive, being in SA) and the man's girlfriend was seated in the passenger seat at the time.

[Bob Kennington]

Hmmm. Maybe the intended target was struck. 

To me, your radiographs indicate a lead (non-jacketed) bullet.

I was just about to comment that I've seen lead  bullets that have penetrated plate glass have an "affect" similar to this  image I found on the Web:

 

(The image is unrelated, and you have to ignore the diagonal line.)

That is, a mirror-flat bright surface upon impact (the round area), surrounded by many other fractures that are also very highly reflective.

If anyone needs an AFTE study subject, it would be interesting to see the effects of plate glass, laminated glass, and tempered glass on trajectory through each. I'd suggest using non-jacketed lead bullets only to see if that mirrored-affect is different for the three different glass types.

[Brandon Bertolli]

Sir, the round is definitely jacketed, but the size of the image I first posted is not sufficient to show it. Here is a magnified view, showing the low density metal of a jacket fragment:



The fine horizontal lines across the entire image are caused by a stationary grid designed to improve image quality by reducing secondary scatter reaching the film. The white band at the bottom is cortical bone of the femur. No bones were struck in this case.

I agree on the glass research. Some people may already be involved in that, and I eagerly await their findings.

[Bob Kennington]

Sir, the round is definitely jacketed, but the size of the image I first posted is not sufficient to show it.

As usual, I'm still puzzled; however, please substitute just sayin'  for my prior indicate.  Hijacking of the thread was not intended by me 

[Brandon Bertolli]

It's not hijacking, your questions and comments are most welcome. You guys have always helped me out with info, and I'll do the same if I can.

The thing that differentiates lead fragments or cores from non-lead jacketing is the radiological density.
An easy test for determining whether something is lead or not, is to try to see bony structures or other anatomy 'through' the metal. If it is a completely white (radio-opaque) fragment then you can bet you have found a lead fragment, or in some cases a large steel insert. This all assumes that the radiographic exposure is not too light overall.
But certainly, in all cases, if you can see anatomy 'through' a metal fragment then that fragment is almost certainly jacketing. It could be steel or copper-based, it doesn't matter. It is essentially a thin strip of low-density metal which has a distinctive appearance radiologically.
The best example to illustrate this is a core-jacket separation. I will locate one of those radiographs from my library and post it here.