I had another thread in July on the trigger pull issue with my .40 Shield. It was very inconsistent in both "roughness" and the trigger pull weight.

Took the advice of other posters and, first, polished all the contact points in the action--which did not really make much if any difference, and second bought and installed this: Apex Tactical Duty/Carry Action Enhancement Kit (AEK) S&W M&P Shield .

Since then the trigger pull is consistent and smooth, somewhat lighter such that I don't have to tug hard with the trigger finger to get it to function--the original inconsistent and unpredictably heavy pull weight affected let-off and therefore accuracy.