The go-to principle of doing gun disarms in Krav Maga is Redirect, Control, Attack, and Take Away. You’ll see it referred to by its acronym, RCAT, in our classes. Many gun disarms that we teach at Krav Maga Reborn in our higher level classes involve grabbing the handgun and redirecting it as the gun most likely discharges.
Some people assume that grabbing a handgun by the slide when it discharges will result in grievous damage to your hand. Although at first that may seem reasonable, it is in fact an incorrect assumption. What does happen is that the gun will fail to go into battery for the followup round, thus leaving the gun useless until cleared.
Here is a video of some guys demonstrating that you can grab a handgun by its slide and not suffer any damage. Keep in mind, this video is to demonstrate the gun grab aspect, not the take away.
As you can see, it’s absolutely possible to grab the slide, the gun fires, and to not hurt your hand.