It is my opinion, in answer to your second question, that rifles and shotguns whose barrels are at least 16 and 18 inches in length, respectively, with folding and/or telescoping stocks, which are fully operable with stocks folded or contracted, and whose lengths are less than 26 inches with stocks folded or contracted, fall within the definitions of 'short-barreled rifle' and 'short-barreled shotgun,' and their sale or possession is prohibited by MCL 750.224b; MSA 28.421(2).
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[url=http://www.ag.state.mi.us/opinion/datafiles/1980s/op06280.htm]source[/url]
I actually had a better description, but couldn't find it.
I guess that nothing is really better than the official opinion, no?
edited to add:
Here it is:
16. Do rifles or shotguns with collapsible/folding stocks have to be registered in Michigan?
MCL 750.222 Yes, provided the rifle or shotgun is 30 inches or less in overall length with the stock collapsed/folded but at least 26 inches and the barrel length is at least 16 inches and 18 inches respectively. Attorney General’s opinion #6280: if the firearm is less than 26 inches in overall length or the barrel is less than 16 or 18 inches respectively, the firearm is considered a short-barreled rifle or shotgun and is illegal to possess. MCL 750.224d There is an exception for short-barreled shotguns and short-barreled rifles which the Federal Bureau of Alcohol, Tobacco and Firearms has found to be a curio, relic, antique, museum piece, or collector’s item not likely to be used as a weapon.
from the [url=http://www.michigan.gov/msp/0,1607,7-123-1591_4654-10953--,00.html]MSP CCW FAQ[/url]