That DNR rule on magazines is ment for a permanently attached magazine not a detachable. But a 'uninformed CO' could still give you a hard time.
From DNR
At all times when carried in or on a motor vehicle, including snowmobiles:
Rifles, shotguns, muzzleloading and other firearms must be unloaded in both barrel and magazine and enclosed in a case or carried in the trunk of a vehicle.
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Michigan Law
THE MICHIGAN PENAL CODE (EXCERPT)
Act 328 of 1931
750.227d Transporting or possessing firearm in or upon motor vehicle or self-propelled vehicle designed for land travel; violation as misdemeanor; penalty.
Sec. 227d.
(1) Except as otherwise permitted by law, a person shall not transport or possess in or upon a motor vehicle or any self-propelled vehicle designed for land travel either of the following:
(a) A firearm, other than a pistol, unless the firearm is unloaded and is 1 or more of the following:
(i) Taken down.
(ii) Enclosed in a case.
(iii) Carried in the trunk of the vehicle.
(iv) Inaccessible from the interior of the vehicle.
Michigan State Police Legal Update 66
https://www.michigan.gov/documents/msp/MSP_Legal_Update_No._66_238184_7.pdf
Transporting ammunition and long-guns
MCL 750.227d describes the lawful way to
transport a shotgun or rifle. While they must
be unloaded, they may be transported in the
same case as ammunition (including loaded
magazines).
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Also note 750.227d will allow you to have a 'Taken Down' AR unloaded and on your passenger seat, no case required.