In 2011, the ATF issued a memo that declared that it is illegal for anyone with a medical-marijuana card to possess a gun or ammunition. ...
The message for Californians who use medical marijuana should be pretty clear. The federal government may go after you for using marijuana, and the penalty can be harsh if you possess both legal (in California) medical marijuana and a lawfully-owned gun.
Fortunately, there is a mechanism for mercy. Sens. Pat Leahy, D-Vt., and Rand Paul, R-Ky., are working together on legislation to grant federal judges more judicial discretion under the federal mandatory minimum sentencing system.
Source: San Francisco Chronicle. Read full article. (link)