You don't have to be precise, just shoot at the upper portion of the victims chest and you'll get most of the pellets to hit. Even in the event that the initial shot doesn't kill them they are weak enough to the point a basic melee will finish them. ADS makes a weapon more accurate and does nothing to multiply the range. Basically damage drop off will not start sooner if you hip fire but the chances of you missing a shot does increase. In the case of Shotguns you're firing multiple projectiles in one shot, aiming or not aiming isn't going to do much to the spread of those projectiles (exception for shot package when that was in the game). The purpose of Rangefinder is to increased the distance the projectiles can travel before damage drop off starts. So if you don't have rangefinder on your shotgun you don't even need to aim it to be effective with it.