Even if you are not planning on carrying it as a concealed weapon, the law requires you to register your handgun (i.e., a gun other than a rifle). For many years, it has been well-established law that certain individuals are prohibited from having a registered handgun (e.g., a convicted felon, a respondent to a personal protection order, etc.).
In Michigan, it is now legal under state law to possess small amounts of medical marijuana if you have been approved for a medicinal marijuana card. However, marijuana – for medicinal and/or recreational use – is still illegal under federal law. (Controlled Substances Act, 21 USC 802) Pursuant to the federal criminal code (See 18 USC 922), it is also unlawful for “an unlawful user of a controlled substance” to buy or possess any firearm or ammunition.
Although the State of Michigan does not consider medical marijuana cardholders to be “unlawful users” of a controlled substance, the federal government views this issue differently. The Bureau of Alcohol, Tobacco, Firearms & Explosives (ATF) has issued a memo (see link below) advising local state police agencies to deny handgun registration permits to medicinal marijuana cardholders.
So in Michigan, the question for law-abiding citizens becomes which do you give up, your pot or your glock?