Soldiers used to struggle with this issue, but someone (probably a Soldier) invented a new type of sling that eliminates it. With the old slings, basically ever

Soldiers used to struggle with this issue, but someone (probably a Soldier) invented a new type of sling that eliminates it. With the old slings, basically every time you needed to use your hands, you had to sling your weapon on your back, but the newer version keeps your weapon suspended in front of you, a little lower than chest level. All you have to do is let go, and your weapon stays exactly where you need it. If a shooter is using a handgun, well with just a little bit of practice, you can learn to draw a handgun in a fraction of a second. Basically, I don't think it's a good idea to assume what will or will not be reasonable in the eyes of a mass-murdering lunatic.