This is too good. You've got two hot chicks shrunken down in front of you, making out with each other. You figure you shouldn't let them have all the fun, so you turn the gun on yourself.
Mike jumps up on your desk before firing the gun, of course. He didn't want to become an inch tall and stuck on the floor while the hot chicks go to town on each other a hundred feet above. The impact of his body landing on the desk scares the girls, probably understandably, and they stop their porn imitation to look at him. He doesn't really mind, as they'll be back at it again soon. He fires the gun.
The impact of the shrinking beam is a lot for Mike to handle, as it's his first time being hit by the gun. He starts to understand a little better why these two women took so long to react when he shrunk them back at the library.
Mike knows what's going on better than they did, though, so he recovers more quickly. Still, although the gun shrinks along with Mike, the sudden force of the beam knocks the gun out of your hand. “As, shit,” he says.