There is a shorter FSB that goes on top of the rail, then the taller one that goes on a weaver gas block, hence if you are using the short one on a gas block, could be the problem.


Then there is the barrel not correctly seated on the upper barrel socket shoulder that will cock the barrel/muzzle off center line of the receiver, as well as the float tube not installed correctly, and the end of it cocked off center line instead.

If you have the correct sights, then suspect that that either the barrel or float tube (if the FSB is mounted on it) is not on correct.

Also to point out, it is possible to cock the upper barrel socket itself from over torque as well.