Bore Offset
One other thing to remember is that the origin of the bullet will be from the actual weapon muzzle, and not the center of the screen as in some games. Because of this, you have to keep in mind that your weapon sights are a few inches above the rifle bore. If you do not take this into account, you will occasionally find instances where you're shooting into the ground (or an obstacle) even though your sights give you the impression that you have a clear line of fire. This becomes more pronounced if you are using the backup sights on a scope. Because of the size of the scope, the backup sights will end up significantly higher above the bore than otherwise, leading to an even larger discrepancy between bore line and sight line.
Note that the Arma 3 crosshair will indicate if something is close to the bore by shifting position backwards to reflect where the bullet will strike. This is an easy way to tell if your muzzle is masked by an obstruction.
