You say it’s CLEAN….but is it clean internally? When’s the last time the gun had a full strip and clean?? Usually the cause of firing pin drag are elongated and misshaped (mushroomed) firing pins/strikers. Happens over time. Might simply be a need for a full strip & clean and lightly reshaping the tips of the pins/strikers…..or if they’re really worn and past the point of a simple reshape…having the strikers welded up and re-shaped.