First I would do as bertram advised and move the target closer to around 10 yards. Then I would Bench the weapon for good support (obviously assuming you do not have adjustable sights) And since you stated you do not have any difficulty hitting a target with your other pistols and calibers we are ruling out pre-anticipation or flinching. Use a small target to aim at 3" circle or bullseye on the target backer. Do your test shots. 4 or 5 slow controlled shots one at a time. I would also check the riflings at the muzzle with a bright light to see if there are any damaged. They all should look uniform! Has the pistol ever been dropped? Did you buy it new? After checking, if nothing is noticeable and you are still hitting ridiculously low. *I would contact Sig Technical Support direct. Who knows? the pistol might have the wrong sights on it? That certainly would explain the problem due to the wrong height of the sight or sights. Sig could no doubt fix anything that is wrong with it. And I am sure it would not be the first time they have ran into that situation. Obviously not worth much now as it is.
The Constitution is not an instrument for the Government to restrain the people. It is an instrument for the people to restrain the Government!
Today, we need a nation of Minutemen, citizens who are not only prepared to take arms, but citizens who regard the preservation of freedom as the basic purpose of their daily life and who are willing to consciously work and sacrifice for that freedom."
-- John F. Kennedy