The sniper rifle butt does the same damage as the slappers. The games code confirms it, a quick test in multiplayer confirms it, and generally every learned GE player confirms it. That being said, the damage these weapons do isn't determined by where you hit the person, it's where you hit them from, so it would be easy to mess up a test determining the damage. A hit from behind will do 2 bars of damage, a hit from the side 1, and a hit from the front will do half a bar of damage. The only difference between these weapons is a slight difference in swing speed and appearance.