6.5 Grendel barrel Length?????

[lazyengineer]

Unattached lower's are marked as "other" on that form... unless the ffl makes a mistake.
Sometimes they ask you while doing their part, I tell them IDK, maybe a pistol maybe an AR... haven't made up my mind. B/c I haven't; after all it is just a piece/part right now. I'm not into SBR's as of now.

Correct, a new AR is now sold on the ATF form as transferred as Rifle, Pistol, or Other. And on that, all stripped lowers are done as Other. Actually, even complete lowers are sold as Other, (if it doesn't have an upper on it). Even if you tell the FFL it's a pistol or rifle or so configured for that half; technically they still have to transfer it to you as Other. So to GreyFox' question - the only ATF official record of a stripped lower, is "Other". So they don't actually know how it started life as a functional firearm; unless you did something to solid document it on your own somehow.