From Guns.com:

Although marijuana is legal for adults in nines states, and as medical cannabis in 30, it remains listed as a highly-addictive and dangerous Schedule I drug with the federal government. It also shares the same designation as ecstasy, heroin, and LSD. This has resulted in the Bureau of Alcohol, Tobacco, Firearms and Explosives maintaining a status quo that any sale or possession of firearms by those who use marijuana, even if allowed by state law, remains criminal.