You want an SG between 1.5 and 1.6 because you want to make sure you are maximizing the BC of the bullet. Bullets can fly accurately, group precisely, and appear to be perfectly stabilized when flying with a SG below 1.5, but can have measurably depressed BC’s. Another reason you want to aim for an SG between 1.5 and 1.6 is to have that extra 0.5-0.6 of SG as a safety margin. If you aim for an SG of 1.0 and conditions are less than optimal, you have a problem.
What SG do I want for my rifle/barrel?