HELENA, Montana — Firearms dealers in states that allow medical marijuana can't sell guns or ammunition to registered users of the drug, a policy that marijuana and gun-rights groups say denies Second Amendment rights to individuals who are following state law. Federal law already makes it illegal for someone to possess a gun if he or she is "an unlawful user of, or addicted to" marijuana or other controlled substances. A Sept. 21 letter from the Bureau of Alcohol,... (GunPolicy.org)