What can I expect when shooting through glass?

First, it is important to note that there are few absolutes and while these situations can be tested in a controlled laboratory setting there is no guarantee that bullets will perform exactly the same way everytime.

The biggest factor in taking a shot through glass is the angle of obliquity, or basically, how perpendicular the bullet is to the glass. Shots that can be taken at an angle of obliquity less than 15 degrees are preferred. At any angle greater than 15 degrees the bullet will generally start to yaw and be deflected. It is important that shots with an angle of obliquity greater than 30 degrees be avoided if at all possible. For angles of obliquity between 15-30 degrees the target needs to be a maximum of 6 feet away from the glass. If the angle is 30-45 degrees the target should be a maximum of 3 feet away from the glass. Shots with angles greater than 45 degrees should never be taken. For nearly perpendicular shots the target can be up to 10 feet away from the glass. Any shot with an angle of obliquity higher than 15 degrees should be taken only if there are no other options.

Shots with an angle of obliquity greater than 15 degrees will generally travel within a cone behind the glass. This results in groups that will grow .4-.8 inches per foot of bullet travel behind the glass.

The debris cloud following a bullet's exit from glass contains thousands of fragments of glass from the size of dust to the size of a .22 bullet. The cloud also contains bullet fragments weighing up to 20 grains with a velocity similar to the bullet itself. Both the glass and bullet fragments are capable of significant penetration. The debris cloud will spread at about 3.5 inches per foot in the first 4 feet behind the glass. The debris cloud must be taken into account in hostage situations. Collateral damage is a serious threat.

The last important issue to consider is the performance of different types of bullets. FMJ bullets are extremely unpredictable and their use is strongly discouraged. Bullets with conventional jacketed design will almost always shed their jacket and typically lose 30-40% of their weight and 40-50% of their length. Bonded bullets will retain more weight and fragment less but they may present severe over-penetration risk. BTHP bullets will be deflected and break up more when they hit glass because there is nothing protecting the general structure of the bullet from deformity. Soft point bullets, like polymer and lead tipped bullets, will usually deflect and break up less because the soft point of the bullet absorbs the force of the impact and prevents the glass from deforming the general bullet structure as severely. Bullets in .22 caliber generally have deflection angles 3 to 4 times greater than .30 caliber bullets because they are significantly lighter.