Pennsylvania defines deadly force as “force which, under the circumstances in which it is used, is readily capable of causing death or serious bodily injury.” It’s important to note that when you fire a gun for self-defense, legally, you are using “deadly force.” If a firearm is discharged, it has the capability of causing death or serious bodily injury, no matter the caliber of the weapon, the type of gun, or any modifications made to the firearm.