Unlawful users of controlled substances are prohibited from possessing firearms, ammunition, or explosives under federal law. 18 USC 922(g)(3). Marijuana is still a controlled substance under federal law. An attorney at the FBI NICS division stated that an applicant having a medical marijuana card is a "slam dunk" for a NICS denial. If that card is registered with the government, there is a good chance that it will be reported to NICS. A NICS denial means that the person is prohibited from purchasing and/or possessing a firearm. You need to consult an attorney.
This is for general informational purposes only. It does not constitute legal advice and does not establish an attorney-client relationship.