Stop! Or My Mom Will Shoot (1992) on Netflix

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 case. Eventually the film draws to a denouement involving the title of the film and a revelation that even though she may seem frail and nosy but she is capable of strong actions in some circumstances, i.e. when her offspring is threatened. And now a cop teams up with his mother to catch the killer from a murder she witnessed.