Azalin:
You're kidding, right?
IIRC the M3 Greasegun was the tank gun of choice before the carbine M16 genre. The M3 replaced the Thompson submachine gun. The military split the difference and went with a new battle rifle (the M1 Garand, correct me if I'm wrong please) and the M3 replacing the Thompson. I'm trying to recall what I read off of a Thompson site, relating the history and development of the Thompson (the M3 was much cheaper than the Thompson).
The M3 greasegun is an open bolt gun and does have a foldable wire stock. There are NO parts interchangable with the AR15/M16 family of weapons that I know of.
Are you thinking of some M16 variant that I've never heard of? If so, please post more information on the rifle as I've never heard of it.
To answer the question, there have been shorty M16's around since Vietnam. Under the general label CAR-15's, they've had 10.5 inch barrels and 5.5 inch flash suppressors. Then there are the Commando type M16's with 10.5 inch barrels and standard A1 or A2 flash suppressors. The 16" AR15 carbine may have been an attempt to comply with the NFA for citizens, as any rifle barrel under 16" requires Class III paperwork.
I believe the actual 14.5 inch barrel and A2 flash suppressor of the M4 came out of a contract for a mideast nation. I believe US later adopted it and added the barrel step to mount the M203.
This is my spur of the moment recollection can bring to the table without any more archives research. Please jump in and correct any errors in my post. I think it's correct for the most part.