6.5 Grendel barrel Length?????

[lazyengineer]

So, Lz, a question... how will anyone at ATF know/be able to prove whether that particular stripped lower (sold as a stripped lower) was "first" assembled into a pistol or rifle by the end user?

Ha! now that's a separate question. Short answer is they won't. Unless you tell them, or you otherwise documented it as starting as a rifle. For the most part, it really only comes into play if you bought a complete rifle and converted it into a pistol.