It is not unusual for police officers to face a threat behind glass. In desperate situations where numbers of innocents are being killed in an active shooter event, glass doors or windows are about the easiest ingress point and can be breached in several ways.
Reports from the Sandy Hook elementary school attack suggest the killer there used a firearm to breach the glass alongside the doors to gain entry. Patrol officers can use the same entry technique in such buildings.
Obviously, any police firearm projectile can penetrate glass, except that which is specifically designed to be bullet resistant.
There are four different types of glass you may encounter, though only one type is readily apparent.
Most glass doors (and their side windows) are either ordinary plate or tempered glass. Plate glass can be more than one inch thick in some instances, but usually is no more than a quarter inch thick. Any bullet from a handgun or long gun will shatter plate glass, which can result in large, razor-sharp fragments that pose a significant danger.
I once saw a very large quarter-inch plate storefront window survive two strikes from a firefighter’s axe during a fire. After an on-scene police officer put a 9mm round through the window, the next axe strike shattered it impressively.
Tempered plate glass is specially heat treated and stronger than ordinary plate, and when it shatters, it generally crumbles into relatively harmless chunks. Some building doors and side windows are made from tempered glass, as are the side/rear windows in vehicles.
A third variety of glass is laminated. Vehicle windshields are a lamination of two layers of glass on the outside with a very strong plastic layer between. As we know from the FBI ammunition testing protocols, virtually any bullet will penetrate laminated windshield glass, but the bullets are often severely damaged in the process. Even after a bullet penetration, laminated glass is very difficult to breach adequately to get a hole large enough to permit entry.
The only type of glass which is obvious by observation is wired glass. Wired glass is often used in commercial and government buildings where top security is desired — it is very commonly used in schools. Wired glass is easily shattered, but I swear the wires making up the one-inch square grid within the glass are made of Kryptonite. About the only practical way to breach a wired glass ingress point is with an axe to chop around the edges — a time-consuming process.
As a firearms instructor, it is worth your time to put together a demonstration at your next firearms training session. Auto glass companies will give you all the cracked windshields you want to haul away. Build a simple wooden rack to hold a windshield at the normal car height and angle and let your officers engage targets from both directions, shooting in and out at threat targets.
After the Virginia Tech attack we tested and videotaped the use of issue weapons/loads (.40 180gr JHP, 12ga Buckshot/Slugs and 5.56mm 55gr Soft Point/FMJ) against glass samples and several grades of chain and padlocks. As expected, any of the weapons would readily shatter any of the glass samples. When firing through glass to gain entry into a building, we recommend firing at about a 45-degree angle off to one side and the same 45-degree angle down — directing your projectiles into the concrete floor to minimize hazards to both yourself and anyone downrange.
Shooting into the face of a warded padlock (a la Master Lock) will do nothing to breach the lock. Pistol/shotgun fire will only deeply dent the steel laminations. A 5.56mm or larger rifle will punch a hole through the lock (just like in their commercials), but the lock will hold. Shooting down onto the top of the lock will generally blow it free of the shackles, gaining you entry. But, you are likely to catch some “splatter” of bullet and lock fragments.
Breaching chains with gunfire, even extremely close range gunfire, is a matter of chance that the projectile will get enough “bite” to sever a link. Shotgun slugs had the best chance against chains, but even then it may take several shots. The only reliable way we found to defeat a chain is a set of bolt cutters.
If the target building has ground floor doors, drive a car through the damn things if need be. Just make sure you have officers very near to quickly exploit the breach and protect the driver from the internal threat. The driver must also be prepared for the vehicle’s air bag to deploy.
Desperate incidents can justify desperate means to gain access and save lives.