I was just looking at the photo where the slap is happening. It looks like there may be a pistol under the victim’s vest on his left side. Take a look.
I think I see what you're referring to. Victim's left side? I'm not sure there is a firearm there. Dolloff's hand is close though and could have been moved away from the vic's left side by the slap.
That would be even more reason to use the pepper spray and it would explain the 2nd gun recovered.