I don't really have a horse in the race, but I would imagine, that with states starting to decriminalize/legalize marijuana use it's only a matter of time until things wind up at SCOTUS and the federal law gets overturned.
In the mean time if you've had a rough week and need to relax, but don't want to compromise your 2nd amendment rights, stick to booze and/or whatever pills your doctor prescribes. It might be hypocritical, but it's the way things are.
Also, the form says "an unlawful user of...", so if you live in a state where it's legal, or use it medically, couldn't make an argument that you're not using it illegally? Or does the federal law supersede the state law?
si vis pacem para bellum