In the description of the weapons I see range, speed and fire rates. However there has to be a difference in power.
You're right there is a difference in power but you can't find that information in-game. There are a few damage charts around the net that give a very detailed look at each weapons damage and how effects like range and suppression change it.
At the bottom of the detailed chart it also talks about how various ammo types effect the shotguns.