The OP says the gun shoots 12-18 inches low at 25 yds. You would only need one reasonably well-done shot on patterning board to see there is something terribly wrong with the gun if it is indeed that far off. In that case, a return to CSMC is the remedy.
However, the "12-18 inches low" statement makes it sound to me like the patterning board work was not all that carefully done, since a good session at the patterning board can determine average point of impact to within less than an inch (i.e., consistent mounting, careful shooting, averaging a sufficient number of shots, even checking the standard deviation, etc.). Consequently, I don't think we really know how far off the gun is shooting. Until we know the actual amount of POI error, we don't know the best fix.
If it were me, I would redo the patterning session being very careful to get good reliable results before deciding what to do. If you are going to send the gun back to CSMC claiming a large POI error, you would want good solid data to back up your claim, anyway.
JMHO
--shinbone
