The big issue with the laser, and more so on side mounted lasers, is the offset from the bore. I pointed my 12ga boresighter at an object 10' away and lined up the Crimson Trace laser to the same point. Now a target 30' away shows the dots are separated by a pretty good gap (I've long forgotten the numbers- just working from memory).
If your intent is to use the laser to help with getting a slug on the target, keep that in mind. In other words, if your laser is sighted at a certain distance, you will hit targets at that distance, but may miss if the target is nearer or farther. You can train for that I suppose, but it will take a lot of shells. I imagine the issue would be less pronounced with buckshot but I haven't experimented with mine yet.