SALT LAKE CITY — A federal judge Friday rejected a Utah gun rights advocate’s attempt to block a newly imposed ban on an accessory that modifies rifles to fire like automatic weapons. U.S. District Judge Jill Parrish concluded that Clark Aposhian didn’t show a "substantial likelihood" of winning the lawsuit he filed against the government on its merits and, as a result, denied his motion for a preliminary injunction. Aposhian, chairman of the Utah Shooting Sports Council, argues in the lawsuit that the Department of Justice and Bureau of Alcohol, Tobacco, Firearms and Explosives violated the Constitution in prohibiting a device known as a bump stock. Bump stocks came under intense scrutiny after a gunman used them to kill 58 people at a country music concert in Las Vegas in 2017. The Trump administration in late December adopted a new federal rule that redefined the devices as "machine guns," therefore banning them under existing law. The rule directs owners to destroy or surrender their bump stocks to the ATF before it takes effect March 26. Aposhian contends the government is illegally changing the rules in the middle of the game. Parrish found neither the political impetus for the rule nor the fact that it reflects a change in ATF policy undermines its validity.