Tapping on the screen twice with your right thumb allows you to aim down sights, which is of course very useful, and definitely the best way to go for rifles and other weapons. Shotguns, however, will do more damage up close. Interestingly, aiming at your opponent while moving towards them will slow you to walk speed instead of sprint speed so you can hipfire - not always optimal. If you want to get closer with any weapon, don't aim at your enemy. Aim at the side of them, and then aim at them when ready to fire. Best tactic to use with the shotgun, though it can help with the ranges of other weapons too.