Sig sights are set up from the factory to be dead on accurate at 25 yard and the sights on sigs are combat sights so a traditional sight picture will not work or you will shoot low. The correct sig sight picture is below and practice trigger control, grip, and not flinching and you will get excellent groupings
Now after all that if the sights are off then they where never set up at the factory since sig uses a 200 dollar and or so sight pusher to move the sights.
Have and experienced shooter and or sig owner shoot your gun bench rested from about 10 to 15 yards ad see how they do. Modern day firearms are more accurate then humans and more then not it's us that are making the mistakes.
Sig 229 owner