Doesn’t MI have one of those weird additional requirements where you also have to register SBRs (and maybe even SBSs) as a pistol if it’s below a certain OAL?
Yep. Things that can be fired in a configuration under 26" have to go on the pistol registry as well. It used to be anything uner 30"... So I have a mossberg 500 with a folding stock, an SBR AR, a beretta CX4, a walther p22, and a 590 AOW that are all "pistols" in MI. I would have bought a few more fun guns to list as pistols, but when they changed the law to 26" on jan 1 2013, sandy hook had just happened and prices shot way the F up.
Anyway ... That means since you can operate an 11.5" SBR AR without the muzzle device and stock, and it comes in at about 25.5", it has to get registered.
However, that means under MI law, your SBR is still a pistol so you can drive around with it loaded in your passenger seat. If you go camping in bear/lion country up in the UP, you can go heavy in and out of vehicles.
ALSO, because the pistol registry form, the MI RI060, has spaces for the name of the buyer and the seller - if you build your own, you're not technically the buyer OR the seller. The state police tell you you need to just put your name in both, or list "self made" for one of them, or do whatever you can to get your gun registered... HOWEVER, that would technically be knowingly submitting false information on a state legal document - a felony. Failure to register a pistol is a civil infraction that is punishable by a $250 fine. also also, people with concealed pistol licenses from other states are exempt from the pistol registry... so if you have another state's out of state carry license AND a michigan concealed pistol license, you can ignore the whole registry fiasco (backed by a published AG opinion).
The good folks at MI open carry, MI gun owners, and MI coalition of responsible gun owners are all working to get those stupid laws straightened out, but it wasn't easy with the last RINO governor and it's much harder with the extra-dem governor right now.