Watch shows like "Lockup" and they use wooden bullets or bean bags from 12GA shotguns, and it leaves welts, probably hurts like a mother, but it does not incapacitate them and the can still continue to attack. With many departments carrying 9mm, it probably doesn't deliver as much force as a 12GA beanbag.
The same rational as why police departments don't carry beanbag as the first round in their shotguns, technology that exists today.
What if in a rush it does not line up exactly on the slide, so the actual bulled hits the hole? In which direction are bullet fragments going to spray from very near the muzzle?