The following is a hypothetical question, but I would appreciate if anyone has real world examples of guns like this that might apply. My question has to do with federal law, not any particular state law, so no CA, MI, NY, etc. specifics apply.
Imagine a bullpup rifle with a 16.5" barrel.
The bullpup rifle has an extendable telescoping stock which is continuously adjustable.
When the stock is fully extended, the rifle is 26.5" length from the muzzle crown to the stock end along a line parallel to the bore. When the stock is fully collapsed, the gun OAL is only 22".
The rifle can be fired when the stock is fully extended, fully collapsed, and anywhere in between.
Am I correct in assuming that this gun is NOT a short-barreled rifle (according to federal laws)? If not, why not?
Federally you are correct, because ATF specifies that the measurement is made with the stock fully extended.
Some states, like California, require that the measurement be made with the stock collapsed, if the gun is capable of being fired in that configuration - so you'd be wrong in any jurisdiction with its own, tighter laws.
The local laws could make your possession possibly illegal, but would not result in the item being an SBR under federal law.