Fundamentals of Shooting Through Glass

This post is written for the Special Operations Police in Ankara, Turkey as a follow-up module to the long/extreme range, soft-target interdiction course I taught them, which covered ranging, wind, moving targets, downing drones, loop-hole, and other techniques used for long-range and extreme range shooting.

Teaching Special Operations Police in Ankara, Turkey

Since the dawn of automobiles, Law-enforcement officers have been required to exchange fire with armed criminals in moving vehicles. As time has passed, the advent of hostage situations and other criminal activity, also has given birth to special operations teams, especially in relation to terrorism, and similar units to shoot through the glass of buildings, homes, and industrial facilities. Like the refraction of a stick under water, so too are there similar effects of bullets passing through glass under conditions that depart radically from those of ordinary bullet trajectories. Such shots often look like the following picture which is that of a sniper bullet through a campaign office window in Hawaii.

Although it is occasionally necessary to fire through glass windows, either in buildings or in automobiles, the art and science of shooting through glass remains underappreciated. This is partly because of the nature of such shooting. If you do shoot through glass, head shots to immediately incapacitate a criminal or terrorists and to limit collateral damage, are not ideal. The deflection of the bullet changes quickly from the angle of the shot taken and the type of glass you are shooting through.

Some of the consequences of shooting through glass are deformation of the bullet. For example, a soft nose or hollow point bullet, especially from a pistol, will generally result in excessive deformation, which will have a detrimental effect on bullet stability in flight. This effect on stability will affect any and all subsequent penetrations of the bullet after it has passed through the glass. Consequently, jacketed or solid bullets seem best at shooting through most types of glass. Also, loss of energy and momentum of a bullet after passing through glass is greatly affected. As an example, the work expended by the bullet as it punctures the glass results in a direct loss of energy that increases significantly with the thickness of the glass. This loss of energy, combined with bullet deformation, reduces the over-all effectiveness of the bullet. Because of this, a bullet diameter that is equal to or greater than the thickness of the glass you intend to shoot through is recommended.

Another consequence of shooting through glass is deflection of the bullet, which must be considered in both directions – the range of the muzzle from the glass and the subsequent distance of the target from the glass. For a sniper, law-enforcement officer, or special-operations agent, this factor is of important practical interest. The primary reason of its importance is that deflection of the bullet by the glass will immediately decrease accuracy of the round. Not only are the following concepts important, the type of glass compounds accuracy issues for those required to shoot through glass and effectively engage and immediately incapacitate a target.

Types of Glass
Whether you have a vehicle, coffee table or industrial building or skyscraper, glass is prevalent everywhere. Further, because criminal and terrorist activity is becoming more intense in larger cities, i.e., Nice, France, London, etc., it is important to know the type of glass you’re dealing with and its characteristics since the latter seriously affect accuracy of the shooter.

It is difficult to argue the facts that glass is reflective, transparent, and reasonably strong (depending on the type of glass). Luckily for shooters, strength is not the only thing that varies depending on how the glass is made; there is another rather unusual trait that separates certain glass types from others – the way the glass breaks and this is what you as a shooter need to understand.

If you have ever had hail break your car or house windows or if you have ever watched movie scenes where automobiles get smashed, you likely have noticed that the glass in car windows does not break like regular glass. Instead, it breaks up into very tiny, granular pieces, whereas a house window will break in larger fractured pieces. Why does this happen? It is because of the different types of glass.

Tempered glass
Tempered glass can be assessed by its appearance and how it feels to the touch thus, it is easy to tell the glass they use in automobiles apart from regular glass. The former is called tempered or toughened glass, and is used in shower doors, refrigerator trays, architectural glass doors and tables, bullet-resistant glass, and smartphone screens. It is known to be 4-5 times stronger than regular glass and can withstand temperatures up to almost 300 degrees Celsius. In addition to strength, tempered glass is known for the characteristic way it breaks. As an example, unlike regular glass, which shatters into sharp shards that can often cause injuries, tempered glass breaks into smaller pieces that interlock with neighboring pieces and therefore do not fall readily, i.e., tempered glass does not shatter out of the frame; instead, it converts into tiny granular pieces and remains intact.

This is why sniper and marksman teams are taught not to make a shot through glass unless they have a straight-on-shot and the target is close to the medium. When they are required to make a shot at an angle, it is generally recommended that the team consider using one shot to fracture the glass and a second to hit the criminal/terrorist suspect. This a process called simultaneous engagement and requires excellent timing. It also must ensure that the extra round does not penetrate hostage(s) unless you are willing to accept a certain level of collateral damage. An additional unknown is the effect of this process is secondary projectile glass that can become lethal after being penetrated by a bullet. However, I have observed through experience that if the target and or hostage is five feet or less from the glass, a potentially maiming/lethal shower of glass shards from 4-9 inches in diameter will be hurtled toward the target and or hostage as the bullet penetrates the glass.

You should also be aware that not all tempered glass in all countries is manufactured to the same specifications, even within the same country. Consequently, since tempered glass will not fall from the window/door frame, I have found that shooting an edge of the frame that has contact with more of the glass in a linear fashion is the best place to place the first shot as it will more often fracture the entire glass in the frame, which is ideal for a second shot from another marksman, i.e., simultaneous engagement. I have had tempered glass not fracture when attempting to shoot a corner to initiate action for the second shot. Further, the average thickness of tempered glass is ¼ inch but, it comes in varied thicknesses depending on need.

As mentioned earlier, the characteristics of any glass type depend on the materials it is made with and the chemical and mechanical processes it undergoes during its manufacture. The way tempered glass breaks into much smaller pieces is attributed to the manufacturing processes in which nickel sulfide inclusions are tempered by heating followed by rapidly cooling to room temperature.

Cooling takes place much faster at the center of the glass than at the surface. This causes compressive stresses in the surface, whereas contraction and tensile stresses are caused in the center. The compressive stress gives toughened glass its increased strength; the higher the stress, the smaller pieces that the glass breaks into. The benefit of differential heating comes in the form of increased endurance of the glass and allows the glass to be stretched or pulled to a certain limit without breaking, which is a good thing to have in many forms of glass. For toughened glass, the compressive stress on the surface must be more than 10,000 psi, whereas for safety glass, it must be more than 15,000 psi. Additionally, this also means a center shot may fracture the glass more efficiently, but often this is where your target may be and if they have a hostage, may not be the ideal placement for the first shot. The best way to know for sure about tempered glass in your area is to obtain a few pieces or 2×3 to 4×4 feet square, place them in a frame and shoot them in different locations, i.e., center, corner, and frame edge that holds the glass in place to determine nominal fracture for your purposes. This is the only way to really know.

Laminated glass
Glass used in automobiles, trucks, and other vehicles is laminated glass for the windshields. Toughened glass is used in side windows and is different from the glass used in the windshield. Laminated glass is made by sandwiching a laminate (usually polyvinyl butyral or PVB plastic) between two (or more) layers of glasses and bonding them using high pressure and heat. In addition to being very strong, this also offers a unique advantage. Upon breaking, the broken pieces of glass do not usually fall out but stick to the laminate and tend to stay in the frame. Therefore, applying significant force on the windshield will make the whole thing pop out of its frame in a single piece. In additional to windshields, due to their strength and the ability to withstand a lot of pressure, laminated doors are widely used in office complexes and public buildings. Those in schools and other facilities are often reinforces with a wire mesh embedded with the laminate, which resists significant force, as well as offering decent soundproofing. This type of glass is difficult to penetrate without significant deflection of the round. Therefore, engaging a moving vehicle where the windshield is generally already at a steep angle, presents a challenge to the shooter. Laminated glass is familiar to almost all and almost everyone recognizes the familiar rock ‘ding’ in the windshield as pictured below.

Plate Glass
People often refer to any window that is not stained as a plate glass window. Most modern plate glass windows are not plate glass at all. Although materials that go into glass of various types are very similar, the processes by which glass is manufactured are different. Plate glass was the industry standard between the 1920s and the 1960s. Thus, if homes were built before 1920 or after 1960, what you might think is a plate glass window could quite easily be made of a different kind of glass.

Plate glass that was made before 1920 was practically hand-crafted. It was valuable then, and it is valuable today. It is commonly used for large mirrors and windows. The product that was replaced by plate glass’s industrialization, called sheet glass, is also a rarity these days. Knowing what kind of glass is in a home or other building is pertinent for shooters due to obvious reasons.

A new process that uses molten tin to create the surface of a glass sheet was developed in the late 1950s. The glass, in liquid form, was poured onto the tin causing the lighter glass to float. The resulting product, “float glass,” had an extraordinarily smooth surface. Today, glass for commercial use is almost entirely float glass, and plate glass is used only in rare cases. Depending on the thickness of the float glass, it will shatter apart when shot into larger pieces than tempered glass. However, older plate glass will leave a hole much like laminated glass although, with a much larger disturbed area, i.e., fracture lines, around the hole.

Considerations when Shooting Through Glass
One of the questions you need to ask yourself before you shoot through any type of glass is, how much collateral damage is acceptable? This will determine whether you take that shot or not. For most, it will be a matter of operational policy dependent upon liability. With that said, let us look at a basic scenario of shooting through tempered glass.

Shooting Through Windows

You are a Swat-team member called to a scene of a three-story office building where a terrorist has taken numerous hostages. The only shot you were able to set up for is directly across the street in an adjacent building in which you have set up for an almost exact perpendicular shot. This is depicted in the graphic below.

As you assess such a scenario, there is a lot that will go through your mind as the shooter. One of the things that go through the mind of most shooters is the burning question, Am I ready to do this?

This is a very valid question because there are so many complexities involving such a shot. The type of glass, type of bullet, potential collateral damage, deflection of the bullet, potential deformation, distance to glass from the muzzle, distance of the target from the glass, and so much more.

Whether you have trained to shoot for this or not, due to the complexities, taking the shot will make you question yourself initially. But, as you settle into the situation, your shooting instincts will prevail as you calm yourself and think logically through your firing process.

One of the You will note that the graphic displays distance of the target from the tempered glass in feet at the bottom of the grid. At the top of the grid, the graphic displays the average deflection of the bullet in inches at each 10 foot distance from the glass.

This deflection is for a perpendicular shot to the glass, i.e., straight on. At a 60 degree angle, the deflection will be about another inch at each distance; at a 45 degree angle to the glass, the deflection is about 3-4 times that listed for each distance. The deflection is measured using a 7.62×51 NATO (.308), 175 grain, Federal Gold Medal Match from an Accuracy International AXMC rifle; .308/.338 calibers.

In a general statement of observation, as long as the target is within 15-20 feet of the glass, a zero hold is warranted and is likely that the target will be struck in a manner and location that will immediately incapacitate the individual. Observation also demonstrates that a center window shot, if possible, will eliminate the need for simultaneous engagement. However, preparations for such would be wise, especially if the angle to glass is less than 60 degrees.

There is no substitution for actual practice, although, tempered glass is expensive to shoot at, along with car windshields, etc. I am often as if there is an equation you can use for this, well, there is, but practice may prove more efficient than the equation once you get the hang of it.

Basic Equation for Shooting Through Flat Glass

The equation assumes you are shooting through non-curved, flat-paned glass, not a windshield and comes from an adaptation of Snell’s law utilizing full, metal jacketed bullets.

Bullet Deflection Angle = INVSIN(TANi x A x T/e^2)

Parameters of equation: i is incident angle, A is density of glass x a/M x Cd where a is the frontal cross-sectional area of projectile; M is mass of projectile; and Cd is the ballistic coefficient. Also, e^2 = 7.389056 (rounding to 3 decimal places is sufficient). For a soft-nosed bullet or bullets that deform on impact, the equation changes to: Bullet Deflection Angle = INVSIN(TANi x A x T/e^2) + (90 degree deflection angle). Beware that the bullets energy is further changed by deformation and potential loss of mass while penetrating the glass, so, this formula is an estimate that should be verified by collected data from your shoots.

Shooting Through Vehicle Windshields from the Outside
From experience, most shooting through vehicles is from the outside in (90 percent or more), which is reasonable for special operations and law-enforcement work. The statements in this section are general since a windshield is typically curved from top to bottom and side to side. One of the advantages of shooting through windshields is the limited distance of the target from the windshield. In most instances the target is either behind the steering wheel or in the front passenger seat. Because of this, the distance from the glass to the target is rather consistent from vehicle to vehicle, varying no more than about 2 feet. Additionally, a shower of deadly glass shards are propelled toward the target as the bullet penetrates the glass.

The same rule applies as for rifles when shooting through a windshield. Although a .22 bullet will go through a windshield, it is recommended that your bullet diameter be equal to or greater than the thickness of the glass – greater than is best.

When shooting from the outside of the windshield to the inside into the occupants, the bullet will generally be deflected downward, but not by that much. The average for a pistol bullet is about a 2 inch downward deflection if you are within 5-10 yards of the vehicle. However, due to windshield curvature from side to side, you can also have about a 2-4 inch sideways deflection. If you are using a 7.62 x 51 rifle (.308), the deflection, due to energy and speed of the bullet, is minimal at short ranges. A center of mass hold is adequate for targets behind a windshield. The combination of flying glass and the bullet is usually stop the target immediately.

Shooting Through Vehicle Windshields from the Inside
This type of shooting, from behind the windshield to the outside, is most often done by close-protection teams such as those I recently taught in Istanbul. And, it is quite a bit trickier than shooting through the glass from the outside. I would definitely advise wearing sunglasses or other glasses if you think you may be involved in such as a situation as a close-protection agent, i.e., bodyguard! It’s also a team effort if you’re ambushed, which I will not discuss at this time.

Due to the angle of the glass when shooting from inside out, i.e., downward and away from you, the tendency is for the bullet to deflect upward by as much as 6 inches and can often go right or left missing the target. To shoot effectively from the inside of the vehicle, aim at the target making the first hole in the glass. Then, try to aim such that each successive bullet passes in close proximity through this hole. If you find yourself in this situation, immediately action is necessary or you’ll never make it out of the vehicle, especially if it is stopped.

The key is to get the target to duck your bullets as you exit the vehicle if stopped so that you can get a better shot. This is something you’ll need to practice and, it’s a process that dynamically changes by the second. Again, the key is to aim for center of mass – this will give you the best chance of survival!

An obvious conclusion is that firing through glass is inaccurate and not always effective, but understanding the process will improve your accuracy. And, due to the increase in criminal and terrorist activity, the skill set has become more necessary.

In my observations, a distance of muzzle to glass from 100 yards away doesn’t seem to make much difference to the above values. Rather, it is the angle of incidence of muzzle to glass that becomes more important. Good accuracy is achieved between 60-90 degrees incidence angle. As the angle drops toward 45 degrees, the deflection can double and even increase beyond that. From 60 to 90 degrees, deflection can generally keep groups between glass to target withing +/- 1-2 inches: adequate for a lethal shot, as long as the distance from the glass to the target is not more than 30 feet. A real problem is developed if there is a need to puncture additional barriers or material between the glass and the target beyond it. The effectiveness will depend on the material and whether or not the target is directly behind it, but that’s another issue not dealt with here.