18 USC 922
(g) It shall be unlawful for any person—
(3) who is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802));
to ship or transport in interstate or foreign commerce, or possess in or affecting commerce, any firearm or ammunition; or to receive any firearm or ammunition which has been shipped or transported in interstate or foreign commerce.
If you use or are addicted to a controlled substance, as defined by federal law you are prohibited from owning a firearm. Since federal law doesn't recognize medical marijuana being a medical marijuana user and in possession of a firearm would place you in violation of federal law.
While the current justice department says they won't go after medical marijauna users have they made any similar statement about not going after marijauna users who possess firearms.
Right or wrong that is the law. If you choose to violate it you do so at your own risk.