Sure, but read post 11 again. If they're going after you for having bought a gun, they're going after you because you've lied on the 4473. That's a crime.
They know you've lied because you've admitted to being a pot user by getting your state medical marijuana card. Now it will be up to them to prove their case, but all they have to do is show the jury that you have admitted to being a user of the drug (or intending to start using that drug) by applying for the MMJ card. I guess you could try to avoid conviction by claiming that you lied to your state when you got your MMJ card and you really weren't a user and never intended to become one. But that's not going to impress the jury very much. Especially as they'll probably get a warrant and find controlled substance material in your possession.
The crux of the matter ISN'T that you are prohibited because you've been convicted of a crime, or even that you COULD be convicted of a crime. You are prohibited because YOU SAY you're a marijuana user. And, as a marijuana user you CAN'T honestly fill out a 4473 form, period.
You lied, that IN ITSELF is a crime, there's a conviction right there if they seek to prosecute.
Now, beyond the question of purchase, what about possession of a firearm? By your own admission you are a marijuana user. Therefore, (unless you're lying about being a pot user, and getting the state card for absolutely no reason at all!
) it is against the law for you to be in possession of a firearm. See: 18 U.S.C. § 922(g)
To charge you and prosecute you for that they have to have some evidence that you are a) a user of a controlled substance, and b) you're in possession of a firearm. Their first piece of evidence to prove 'a' is that you've obtained a MMJ card! Then, seeing as this is a criminal prosecution, they'll probably get a warrant and search to discover that, yes, you are in fact in possession of and using that drug. Therefore, you have violated 922(g).
This isn't a situation of the government denying you due process. This is a situation of your state government giving you the means to voluntarily disclose to the federal government that you are a prohibited person and may not lawfully buy or possess firearms.
It is a really fascinating situation. By all logic we've created a situation under which we've convinced people to rat themselves out for prosecution and rescinding their own rights. People THINK that having a state law say "it is legal under state law to possess and use pot" means it is LEGAL. But it is still COMPLETELY illegal under federal law, and the 4473 form is simply one manifestation of how federal law WILL affect you personally if you decide to violate it.
This is entirely different from having a secret list of people who might be bad people but who haven't done or admitted to anything we could even start to prosecute them for and denying their rights based on what they MIGHT do someday, we think.