It's going to kick, but remember the that the "shot" is already going to be coming out the end of the barrel when it does. Are you not planning on placing the barrel well into your mouth?
If I were going to use a shotgun, I'd place the butt of the gun on the floor, lean forward over the barrel and move my mouth onto the barrel, then reach down and squeeze the trigger. This way you're not really holding the gun, it's being supported by the floor and your mouth.
Here's a gruesome pic of someone who used a shotgun, but apparently did not place the barrel into their mouth, but to the side of their temple instead.
WARNING it is very gruesome. I don't know if any of the picture is photo-shopped or not.