Delivering what aides called a general-election-launching speech to the National Rifle Association, Mitt Romney tried to shore up his support with gun owners by warning darkly of President Obama’s authority to appoint Supreme Court justices. Romney painted a picture of a tyrannical president who, in his second term, would be likely to appoint another liberal justice hostile to gun rights. “In his first term, we’ve seen the president try to browbeat the Supreme Court. In a second term, he would remake it,” Romney said. “Our freedoms would be in the hands of an Obama court, not just for four years, but for the next 40. That must not happen.’’