As we’ve pointed out frequently in the past, marijuana may be legal in your state, but it’s still a Schedule I controlled substance as far as the federal government is concerned. That means you’re violating federal law by using it while owning firearms. And if you answer the questions truthfully on a 4473 form, users can’t legally purchase a gun.
[W]hile some states have decriminalized even the recreational use of marijuana, the federal government has not. 21 USC 802(16) lists marijuana (spelled marihuana in the law) as a controlled substance, and it is listed among such substances that can get a person a felony drug conviction. Such a conviction means good-bye to your Second Amendment rights.
Under 18 USC 922, though, one doesn’t need to be convicted on a drug charge to get hit with a 10-year federal sentence. All one has to be is an unlawful user of a controlled substance. This can include marijuana, as this case in Iowa illustrates. The United States Concealed Carry Association has outlined a lot of that on its site.
Here, a number of normal Second Amendment champions have not really been moving on this issue. But Representative Earl Blumenauer (D-OR) has introduced HR 420, the Regulate Marijuana Like Alcohol Act. This legislation takes marijuana and puts it on the same tier as alcohol with regards to federal regulation. The most important feature is that marijuana no longer becomes a controlled substance, which ends a lot of legal jeopardy for those who exercise their Second Amendment rights while using marijuana.
– Harold Hutchison in Below The Radar: Regulate Marijuana Like Alcohol Act