Guns are only efficient in the hands of efficient killers.
It takes a person who knows how to operate a gun to kill people. In order to operate these high-powered, military- assault weapons, it takes a person who has somehow managed to obtain a gun and is comfortable firing it.
The problem is the systems that allow unfit people to do this. In the United States, among those who are not allowed to purchase or possess a firearm are people deemed a danger to society and patients involuntarily committed to mental institutions.
Thus, guns are supposed to be kept of the hands of people who are unfit to operate such dangerous weapons.
But, there is an issue with the system, with the rules and regulations that do not keep firearms away from unfit people who are then able to learn how to operate these weapons.
It is not guns that make killing a lot of people easier and efficient; it is insufficient laws and a broken system that needs to be addressed.