If you hold a medical marijuana card and you walk into a gun store to purchase an AR-15 — or any firearm — under current federal law, you are prohibited from completing that purchase. That's true regardless of whether marijuana is legal in your state, regardless of whether your doctor recommended it, and regardless of how long you've held your MMJ card. That answer frustrates a lot of people, and understandably so. Medical marijuana is legal in the majority of U.S. states. Millions o …