Directors often bristle at the ratings their movies are given by the Motion Picture Association of America. Sometimes, as in the case of Mike Birbiglia’s new movie, “Don’t Think Twice,’’ the filmmaker’s objection is understandable. The MPAA slapped an R rating on the Shrewsbury native’s funny film about a New York improv troupe but gave the big-budget orgy of violence known as “Suicide Squad’’ a PG-13 rating. That isn’t sitting well with Birbiglia, who tweeted: “Suicide Squad has machine gun killings and bombings and got a PG-13 rating. @Dontthinkmovie gets an R because adults smoke pot. Confusing?’’ (The MPAA cited “language and some drug use’’ in giving “Don’t Think Twice’’ the R rating.) Director Judd Apatow, whose films have sometimes included pot-smoking and thus received an R rating, is sympathetic to Birbiglia. “Studios own the ratings board. Violence sells so they make pot and sex the scary thing so they seem caring,’’ Apatow tweeted. It’s likely that folks familiar with Birbiglia’s earlier work, including his appearances on “This American Life’’ and the movie “Sleepwalk With Me,’’ will see “Don’t Think Twice’’ despite the rating.