There is now a question on the Form 4473 that 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.
So, if you have a medical marijuana card, and you tell the truth on your form 4473, you should expect to be denied.
(None of us on this forum, of course, would ever lie on a Form 4473).
Also, I am informed by my attorney that possession of both
any illegal drug
and a firearm is still a felony in Delaware, with or without a CCDW permit and/or a medical marijuana card.