Shooting a bullet through glass isn’t as simple as you might think. I’m sure you’ve seen some videos where cool guys with beards are telling you that bullets always do this, or always do that, when shot through glass. While these rules of thumb can be handy, they sure aren’t accurate. This article is going to cover some interesting research that was used to develop a mathematical model to predict bullet deflection as it passes through glass. I’ve also included a simple spreadsheet that will help you with your bullet calculations. Keep reading for more.
Shooting through glass may not seem like a common problem for most shooters, but if recent events are anything to go by, then it might be more of a consideration than you might think. I want to be clear up front. There are so many variables that affect a bullets behavior through glass that no one can tell you exactly what will happen. However, thanks to Mr. Henri Lambert, we have a solid mathematical equation that will give us a good guess as to what will happen.
Mr. Lambert wrote a research thesis entitled, “The Effects of Commercial Tempered Glass On Rifle Bullet Deflection.” In this thesis, he examined the effect that commercial tempered glass had on .308 caliber rifle rounds. Here’s a quick overview of his research findings.
Shooting a Bullet Through Glass Research Findings
- Shooting through glass at less than 4 yards distance is very unpredictable
- Shooting between 4-100 yards from glass yields predictable deviation
- Shooting at a perpendicular angle is best for engagements through glass
- For angles greater than 15 degrees from 90, the jacket will be stripped from the core
This research thesis was restricted to standard .308 jacketed bullets. The author didn’t test pistol calibers or different types of rifle rounds. Obviously, we’d like to have this test data, but shooting through commercial tempered glass get’s expensive. Let’s cover some facts and fallacies about shooting a bullet through glass.
Shooting a Bullet Through Glass: Facts and Fallacies
Contrary to popular belief, shooting at closer range with rifle cartridges, doesn’t mean you’re going to be more accurate. If you read my article on choosing the best 5.56mm cartridge for self defense and CQB, you’ll remember this graphic
This graphic highlights that bullets generally have a high yaw rate when they first exit the barrel. This data is for 5.56, but the same concepts apply to the 7.62 round as well. The author found that when shooting a 7.62 cartridge at closer than 4 yards to tempered glass, it actually caused a lot of destabilization.
In fact, the author noted that at very close ranges, and extreme angles to glass (45 degrees) the jacket and the bullet core always separated. This separation caused great inaccuracy at a target that was standing 15 feet on the other side of the glass.
In the graphic above, you can see that the jacket and core had approximately a 10 inch spread over three shots. If the average width of a man’s shoulders is 19 inches, then you really have very little margin for error here. Based on this data, it looks like the extreme yaw angles of the bullet caused unpredictable, and unacceptable accuracy, even at close range targets.
Not only did the bullets disperse laterally, they also dispersed horizontally at very close ranges. Here is a great excerpt from the research paper, highlighting the authors findings.
Every round fired at 45 degrees glass angle resulted in separation of the bullet
jacket from the core, complicating trajectory and increasing the dispersion of the
bullet fragments. The bullets fired at two yards were disbursed over 12 inches in
the X direction and over six inches in the Y direction. At four yards, the bullets
were dispersed at less than five inches. At six yards, bullets were disbursed almost
exactly five inches in the X direction and less than four inches in the Y direction.
At 10 through 50 yards, the dispersion remains nearly constant. This establishes
that the minimum stability distance lies between two and four yards. There is a
possibility that the minimum stability distance lies slightly closer than four yards.
However, if one increases the measured variation by the width of the zero group as
an error factor, the variation slightly exceeds five inches. Therefore, the minimum
predictability distance can be effectively located at four yards.
The Effects of Commercial Tempered Glass On Rifle Bullet Deflection
Base off of this data, we know that close range engagements are going to be extremely problematic, especially if the target is deep into the structure. Let’s take a look at what effect glass has on bullets a little further away.
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Glass Effects on Bullets: Medium Distance Engagements
The author spent the bulk of his time doing these tests. All tests were similar. He engaged a target that was a standardized distance (15yds) behind a pane of glass rotated at 90, 60, and 30 degrees from perpendicular. The author stated in his paper that accuracy outside of a 5 in group would be deemed unacceptable. He found that even at 45 degree angle from glass, most fragments landed within a 5 inch dispersion pattern. There was a notable shift to the right due to the bullets spin as well.
As a quick rule of thumb, we can see that both the bullet core, and jacket hold a pretty tight pattern. We should also remember that 45 degrees caused the largest dispersion compared to less severe angles. If you’re a nerd like me, and you want to know how to predict this dispersion, then you should check out the formula below.
I’m not going to lie, it took me like an hour to decipher all of this. Don’t judge me! I will briefly describe the variables that the equation takes into account. I will also provide a Dropbox link where you can download a copy of the spreadsheet further on.
Mathematical Variables That Affect A Bullet Passing Through Glass
There are really two types of variables that will affect a bullet shooting through glass. There are variables that you won’t really change, and are constant, and there are variables that will change, like bullet weight. The screenshot below is a quick view of the calculator. You have to fill out the boxes in green, and the calculator will give you the deflection angle and the deflection distance.
In this calculator, the bullet cross sectional area is auto calculated from the bullet caliber you enter. The glass density is generally a constant for commercial tempered glass. If you have a special glass, and know the density, you can change it.
Once you enter the mass of the bullet in grains, the spreadsheet auto calculates its weight in pounds. You will also need to get the ballistic coefficient from the bullet manufacturer and enter that. The glass thickness is a variable you can change should you desire.
Lastly, you need to put in the incident angle. The incident angle is zero degrees if you fire perpendicular to the glass. It would be 90 degrees if you fired parallel to the glass. You will also need to input target distance from the glass in inches. Once you’ve entered all the data into the green sections, the calculator will spit out your results.
DOWNLOAD THE BULLET GLASS CALCULATOR HERE
Using the Calculator to Predict Bullet Behavior Through Glass
If you recall the author stated that the jacket and the bullet core almost alway separate when fired through glass. This calculator won’t give you the exact location of each fragment. The deflection it’s spitting out should be thought of where the bullet core is most likely to go, with the bullet jacket landing a few inches a way from the predicted deflection.
I know some of you nerds are mad that I can’t tell you exactly where every piece of the bullet is going to go. Well I don’t have access to a super computer, and I barely know math, so you’ll have to deal with it! Now, let’s talk about shooting through car windows.
What Happens When You Shoot Through A Car Window?
This is a little more complicated than you might think. As a very rough rule of thumb, the bullet generally rises when shooting from inside out. Conversely, it generally goes down when shooting outside in. However there is a giant caveat to this. Glass on cars, is curved!
Now Kyle Lamb is cooler than I’ll ever be, and he brings up some good points. You can’t reliably predict what the bullet will do because you never achieve a straight on shot through a flat pane of glass. This brings us to our next topic of discussion, tips for shooting through glass.
Tips for Shooting Through Glass
The best tip for shooting through glass is to port the glass first. You can port the glass in a variety of ways. If you are considering a sniper engagement, you can do a dual engagement to eliminate the glass on the first shot, and the second shot is aimed normally at the target.
If we are considering a self defense scenario, where you have to shoot through vehicle glass, you should probably expect to use multiple shots, and create your own hole to shoot through. Obviously I’m not a lawyer and this isn’t legal advice to you need to realize that there would be very few instances when you should shoot through glass. If you’re not sure when you could do this, it’s best not to.
Final Thoughts
I found this research to be fascinating. We have established there is some reasonable predictability when shooting a bullet through glass. We’ve also covered a few methods of getting around the glass if needed. I think the biggest take away from the research is that we should avoid shooting at extreme angles if at all possible. Moreover, we should avoid shooting through glass at very close ranges, as that causes extremely unpredictable shot placement.
If you have any questions or comments put them below. Now get out there and get training!
