The ATF and FBI both know the date the Form 4473 was filled out, and they could easily subpoena the gun shop for more information. The evidence of drug use on Hunter's laptop also has times and dates associated with them: The emails, videos, and other things. A first-year law student could throw together an iron-clad prosecution.
And yes, users of marijuana as well as other drugs are prohibited from even possessing a firearm. "Possessing" doesn't just mean "carrying" one. It also mean "owning" one, whether it's in his car, home, or a shed out back of the house.
"The federal government allows legal states to draft and implement their own marijuana laws. And federal laws are applied against people who own, nurture, or supply large amounts of marijuana.
As far as gun ownership by marijuana consumers is concerned it is the government that takes the lead and the legal states follow.
No gun purchase or possession is allowed to marijuana consumers including medical marijuana cardholders."
There are two sides of the fence, use marijuana and give up gun ownership or give up marijuana and be a gun owner. - Alex Mooney