Why Does My Weapon Keep Missing its Aim?

The most probable reason for you to encounter such thing is that you’re press-holding the trigger/fire button. Depending on the weapon that you are using, automatic weapons like assault rifles usually have this kind of “spread” effect when fired spontaneously (which is about right and realistic). To make them more accurate or “efficient”, you might want to do burst shots rather than press-holding the trigger.

The other thing that you might be experiencing is when you try to snipe someone within the creases of their barricade / walls. Remember that these fortifications have their own HP and regardless if you manage to peep inside the creases of the structure, it wouldn't go through it unless you actually destroy the wall first. The only way you can land a clean shot is when you are at a higher ground than they are - as they become vulnerable when you get past the height of their cover.