See the instructions for Question 21.d if you haven't already. It's on page 5 of the 4473 form.
https://www.atf.gov/file/61446/download#page=5
There are some exceptions indicated. For example:
"EXCEPTION: A person is not prohibited from receiving or possessing a firearm if that person: .... (2) has been convicted of a State misdemeanor punishable by imprisonment of two years or less;"
PS: Not that Question 21.d matters if one is still using marijuana they would have to lie on Question 21.f since MJ usage
is still illegal at the federal level even if legal at the state level.
"f. 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."