Stop! Or My Mom Will Shoot (movie, 1992)

A tough cop's seemingly frail mother comes to stay with him and progressively interferes in his life. She buys him an illegal MAC-10 machine pistol and starts poking around in his police cases. Eventually, the film draws to a denouement involving the title of the film and the revelation that even though she seems frail and weak she is capable of strong actions in some circumstances, i.e. when her son is threatened by thugs and she shoots herself in the shoulder.