A number of years ago an individual who worked for me at that time suffered a mental crisis. He threatened suicide and was preventing his family from leaving their home. BCSO deputy sheriffs responded to a 911 call and the individual hid in a bedroom closet with a shotgun. When the deputies tried to get him to come out and drop the shotgun, he pointed it at the deputies who opened fire with their handguns.
The back wall of the bedroom closet where the individual was hiding was adjacent to the laundry room. The laundry room’s wall was adjacent to the resident’s garage (see diagram).
Reportedly the deputies fired over twenty rounds, three of which struck the individual in the lower abdomen, groin, and leg--he survived the shooting.
Several of the deputies’ rounds penetrated into the laundry room and struck appliances. At least nine of the deputies’ bullets penetrated the closet and laundry room sheetrock walls and three fully penetrated the resident’s garage door (see picture #1 below).
Two of these traveled across the street and penetrated a neighbor’s garage door and one of these penetrated the sheetrock wall in the back of the neighbor’s garage. (see pictures #2 & 3 below). These bullets clearly possessed sufficient mass and energy to cause serious bodily harm or death even after they had penetrated two sheetrock walls and two metal garage doors.
Jeff Cooper’s forth rule of firearms safety is “Know your target and what is beyond it.” This incident certainly brings this point home given the penetration of the 9mm rounds the deputy sheriffs fired.* Since many/most people use a firearm for home defense, understanding engagement angles is very important. A safe assumption would be that any round you fire will penetrate at least two interior walls—perhaps more. This may apply to certain exterior walls as well depending upon the home’s construction. In 2021 a Texas woman fired a rifle through the wall of her home, killing a man who had allegedly been peeping into her window.
Residential sheetrock interior walls are little impediment to most handgun, shotgun, and rifle rounds assuming that the rounds do not strike studs or other items in the wall. It seems that residential metal garage doors and some exterior walls are little better.
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* I have been told that the BCSO uses Speer Law Enforcement 124 grain +P Gold Dot; however, I have not been able directly confirmed this. I welcome correction.