UPDATE: I've been reading more on this topic and discussing it with well-read patients. If you look at the word of the law in Virginia (posted below), it clearly says "any person unlawfully in possession of a controlled substance" is not allowed to possess a firearm. In the state of Virginia, possession of marijuana is legal if you have a medical card. Therefore, according to Virginia law, you can have a gun if you have a medical marijuana card.
Where it gets tricky is what the ATF has to say about it. In 2011, the ATF wrote an open letter stating that any possession of a firearm while possessing cannabis, despite having a medical marijuana card, is still illegal due to it being subject to Federal law. And they explicitly say that it doesn't matter if it is legal at a state level. I haven't found any updates to that yet (if you know of something please post below or let me know). So there is direct conflict here between state and federal law regarding possession.
Purchasing is another issue. My understanding is that you must fill out an ATF Firearms Transaction Record in order to purchase a firearm. On line 11-e, the ATF form specifically asks:
"Are you an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance? Warning: The use or possession of marijuana remains unlawful under Federal law regardless of whether it has been legalized or decriminalized for medicinal or recreational purposes in the state where you reside."
That warning, unfortunately, is the kicker. If you are purchasing a firearm from someone who makes you fill this form out and if you answer yes on this form, you cannot purchase the firearm. Hence, there is a good argument that you can legally possess a gun in Virginia, but it will be difficult to purchase one. I'm definitely not a lawyer, so please don't rely on my interpretation. Do your research and talk to a lawyer if you want to be certain.
Virginia Law
§ 18.2-308.4. Possession of firearms while in possession of certain substances.
A. It shall be unlawful for any person unlawfully in possession of a controlled substance classified in Schedule I or II of the Drug Control Act (§ 54.1-3400 et seq.) of Title 54.1 to simultaneously with knowledge and intent possess any firearm. A violation of this subsection is a Class 6 felony and constitutes a separate and distinct felony.
B. It shall be unlawful for any person unlawfully in possession of a controlled substance classified in Schedule I or II of the Drug Control Act (§ 54.1-3400 et seq.) to simultaneously with knowledge and intent possess any firearm on or about his person. A violation of this subsection is a Class 6 felony and constitutes a separate and distinct felony and any person convicted hereunder shall be sentenced to a mandatory minimum term of imprisonment of two years. Such punishment shall be separate and apart from, and shall be made to run consecutively with, any punishment received for the commission of the primary felony.
C. It shall be unlawful for any person to possess, use, or attempt to use any pistol, shotgun, rifle, or other firearm or display such weapon in a threatening manner while committing or attempting to commit the illegal manufacture, sale, distribution, or the possession with the intent to manufacture, sell, or distribute a controlled substance classified in Schedule I or Schedule II of the Drug Control Act (§ 54.1-3400 et seq.) or more than one pound of marijuana. A violation of this subsection is a Class 6 felony, and constitutes a separate and distinct felony and any person convicted hereunder shall be sentenced to a mandatory minimum term of imprisonment of five years. Such punishment shall be separate and apart from, and shall be made to run consecutively with, any punishment received for the commission of the primary felony.
1987, c. 285; 1990, c. 625; 1992, c. 707; 1993, c. 831; 1999, cc. 829, 846; 2003, c. 949; 2004, cc. 461, 995; 2011, cc. 384, 410; 2014, cc. 674, 719.