In prosecution on charge of possession of firearm as felon, Dist. Ct. erred in denying defendant’s motion to admit expert testimony regarding defendant’s low IQ and his mental illness that defendant claimed played role in his confession to police that he possessed firearm that was located in attic of home of his deceased father. Defendant at trial maintained that firearm belonged to deceased father, and govt. conceded at trial that such expert testimony was relevant on issue of trustworthiness of defendant’s confession. While Dist. Ct., in denying defendant’s motion, found that proposed evidence was not relevant because charged offense was general intent crime, proposed testimony should have been admissible on issue of trustworthiness of defendant’s confession, especially given defendant’s alleged mental deficits. Moreover, error was not harmless, where govt.’s case rested largely on defendant’s confession.