When I purchased my Sig P226 mk 25, my first groups were low...see attached pic. After thinking about it and reading, I think that weapon is a combat weapon and it is intended that the white dot on the front sight be placed over/cover the target...when I did that, my groups went where they were supposed to go.
may or may not be the problem you are experiencing, but that was my conclusion with my Sig
Man once surrendering his reason, has no remaining guard against absurdities the most monstrous, and like a ship without rudder, is the sport of every wind.
-Thomas Jefferson to James Smith, 1822.
We cannot solve our problems with the same thinking we used when we created them.