or would it be considered a short barrled rifle?

It must have no stock and weigh less than 50 ounces unloaded.

According to fed law, once a rifle always a rifle.

Just to be a smartass.........

If you converted it to a machinegun it's no longer a rifle, and would never be considered a rifle again [:)]

No SBR in MI, unless it has historical value, IE C&R.
If I have time later, I'll look up the link.
Also, check out the michigan hometown forum. This has been kicked around a few times in the recent past.

[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]