The law enforcement agency that has jurisdiction over federal firearms laws has been without a permanent director since 2006. This is due in large part to behind the scenes efforts by the NRA to block nominees and keep ATF leadership in a state of flux. Nominating Robert Mueller would provide someone with a track record of law enforcement leadership at the federal level. He is well known to members of Congress and the White House.
If the administration is really serious about dealing with gun violence and enforcing federal firearm laws, they need to nominate an equally serious candidate to be the Director of ATF.