A shotgun is not a rifle and never will be, no matter what modifications are made. It will still have too much recoil, too little accuracy and too little capacity. It is NOT shot the same as a rifle either, despite the fact that both are shoulder fired. This is a common error made by people who then complain that their shotgun cannot perform as well as an M4 as their M4 can.
What a shotgun can do well is hit moving targets quickly at short range and do so with a lot of force...unless you tighten the choke to the point you are shooting a slug, put rifle sights, pistol grip stocks, etc to the point that it can no longer be shot as a shotgun should be.
Personally, I disagree that a shotgun is a versatile weapon. I tend to think that it is a very specialized weapon that performs a few tasks well and fails miserably at others.