A short barreled rifle is defined in the US Code, Title 18, Part I, Chapter, Sec. 921.
(8) The term ''short-barreled rifle'' means a rifle having one or more barrels less than sixteen
inches in length and any weapon made from a rifle (whether by alteration, modification, or
otherwise) if such weapon, as modified, has an overall length of less than twenty-six inches.
Notice this doesn't say anything about whether or not it can fire while folded. That is an ATF interpretation that, even from them, gets fuzzy.
Whether or not you'll hang in court remains to be seen I suppose, like many other ATF "rulings".
Here is an interesting opinion from the Michigan Attorney General as it regards those rifles that have at least a 16 inch barrel but are less than 26 inches when folded AND are still operable.
http://www.ag.state.mi.us/opinion/datafiles/1980s/op06280.htm
As in the case of the Kel Tec above, the interpretation here takes into account the portion of US Code that says "whether by alteration, modification, or
otherwise". Since the Kel Tec is manufactured that way, it is not "altered or modified" so it's not an SBR. It
appears from all of this that if the manufacturer made it that way it's fine but if you modify something to end up that way it's an SBR.
The firearms which are referred to in the second question will fall within the definition of a short-barreled rifle or a short-barreled shotgun only if they are considered to have been made from a rifle or shotgun 'by alteration, modification, or otherwise' and are capable of being folded or contracted to less than 26 inches in length.
So, you could have 2 identical rifles, one from the factory and one modified from a longer version, and one would be perfectly legal and one would not.
Now, that's just Michigan AG's interpretation but it makes as much sense as any other I've seen written down and would explain some of these things, like the Kel Tec, that appear to not meet some definitions ATF has "ruled" over the years. That it meets word for word the US Code and is approved by ATF I think is telling that they are not sure they have a leg to stand on regarding factory made Title I firearms.
Have I mentioned this week that gun laws are stupid?