I am correct that the age is 18 to Carry in Michigan?
Yes
The age to purchase handgun ammo is 21 in Michigan, does this prevent me from owning or possessing handgun ammo? If so, what about hand-loads, Since I legally purchase my own materials and load my own rounds. (Although I would be carrying factory defense rounds.)
This does not prevent you from owning or possessing handgun ammo. It just prevents you from buying the ammo from an FFL dealer just like you can't buy a handgun from an FFL dealer.
What are the stipulations on open carrying? Such as, does the safety legally need to be engaged, can there be one in the chamber ready to fire?
Well if you have a "safety" it can still be on safe with a round in the chamber. But back to more what you're asking, yes you can open carry with a round in the chamber.
Are there any stipulations on the amount of ammo, magazines, or the magazine size you are allowed to carry?
Nope, you can carry around 100,000 with you if you so please (might get a bit heavy and expensive).
Does a separate pouch/holster carrying an extra clip also need to be Open or can that be concealed?
The extra MAGAZINE can be open or concealed as there is no law that prevents the carry of concealed magazines.
If I was wearing a short sleeve T-shirt, tucked in, and my arm was at my side blocking view of the gun partially, unintentionally, does this consider it to be concealed?
No.
Also, I drive an S10, being such it has no real "Trunk". Does the locked case for transport need to be in the bed. Or can it be behind or under the passenger or drivers seat?
750.231a Exceptions to MCL 750.227(2); definitions.
Sec. 231a. (1) Subsection (2) of section 227 does not apply to any of the following:
(a) To a person holding a valid license to carry a pistol concealed upon his or her person issued by his or her state of residence except where the pistol is carried in nonconformance with a restriction appearing on the license.
(b) To the regular and ordinary transportation of pistols as merchandise by an authorized agent of a person licensed to manufacture firearms.
(c) To a person carrying an antique firearm as defined in subsection (2), completely unloaded in a closed
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case or container designed for the storage of firearms in the trunk of a vehicle. (d) To a person while transporting a pistol for a lawful purpose that is licensed by the owner or occupant of
the motor vehicle in compliance with section 2 of 1927 PA 372, MCL 28.422, and the pistol is unloaded in a closed case designed for the storage of firearms in the trunk of the vehicle.
(e) To a person while transporting a pistol for a lawful purpose that is licensed by the owner or occupant of the motor vehicle in compliance with section 2 of 1927 PA 372, MCL 28.422, and the pistol is unloaded in a closed case designed for the storage of firearms in a vehicle that does not have a trunk and is not readily accessible to the occupants of the vehicle.
(2) As used in this section: (a) "Antique firearm" means either of the following: (i) A firearm not designed or redesigned for using rimfire or conventional center fire ignition with fixed
ammunition and manufactured in or before 1898, including a matchlock, flintlock, percussion cap, or similar type of ignition system or replica of such a firearm, whether actually manufactured before or after 1898.
(ii) A firearm using fixed ammunition manufactured in or before 1898, for which ammunition is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.
(b) "Lawful purpose" includes the following: (i) While en route to or from a hunting or target shooting area. (ii) While transporting a pistol en route to or from his or her home or place of business and place of repair. (iii) While moving goods from 1 place of abode or business to another place of abode or business. (iv) While transporting a licensed pistol en route to or from a law enforcement agency or for the purpose of
having a law enforcement official take possession of the weapon. (v) While en route to or from his or her abode or place of business and a gun show or places of purchase or
sale. (vi) While en route to or from his or her abode to a public shooting facility or public land where discharge
of firearms is permitted by law, rule, regulation, or local ordinance. (vii) While en route to or from his or her abode to a private property location where the pistol is to be used
as is permitted by law, rule, regulation, or local ordinance.
You may put it (unloaded) in a case designed for the storage of a pistol as long as it's out of you're reach. You are not required by law to put a lock on it but it may be a good idea in case someone tries to say you weren't legally transporting it.