After talking with people that knew more about firing real shotguns than myself I proposed the above thought. The shotgun becomes better when your firing at someone in the 6' to 90' (depending on the shotgun) range. After that the damage is reduced from what it is normally. A sniper rifle would be much better unless you were up close and personal with something as most rifles first range increment would be 90'. And even with the +3, +2, +1...remember that it would equate to +3, zero (+2, -2 for range increment), then -1, then the standard -2 tacked on for each increment. Along with that I'm proposing a loss of damage as the shot goes farther spreds a bit more and looses the power a single slug/shot would have.
If your using a slug, you wouldn't get the benefit to hit at close range.
Tellerve