That makes sense but in the rare situation where you were questioned if both the FFL and Manufacturer marked it as a rifle how do you even begin to prove it was never assembled as a rifle?
I think “rare” is an understatement. First, I’ve never heard of a situation where anyone was questioned about (let alone charged for) the assembly history of a lower receiver they bought. Second, I’ve never seen (or heard of) a receiver coming from the factory labeled as a rifle. Third, most dealers would still enter it into their books as a receiver anyway, since that’s what it is. And finally, any salesperson who bothered to read the instructions on the 4473 would know that a receiver should be marked as an “other” and a “receiver”, not as a “long gun” and a “rifle” (sadly, this one is less rare than the others).
But let’s pretend that all four of those events actually happened together. So what you’d be tasked with is proving that you bought the firearm as a receiver and not a rifle. Remember, it doesn’t matter if it has been built as a rifle at some point, the only thing that matters is that it wasn’t first assembled as a rifle either by you or by the manufacturer. It’s pretty hard to prove how you first built your receiver unless you have videos or pictures, but I’d think there would be evidence that you didn’t buy a full rifle from Aero Precision, you just bought a lower:
-They come in small boxes, so if you still have the box, that’s evidence you didn’t buy a rifle.
-The receipts of sales from the manufacturer to the dealer and from the dealer to you would be for a price that was much lower than the cost of a full rifle. (And if the dealer wasn’t direct with Aero and bought it through a distributor, that’s a third receipt.)
-Any camera footage from the store or from the parking lot would clearly show you leaving the store with a small, non-rifle sized bag.
My point here is that you bought a receiver and there’s plenty of evidence that you bought a receiver, so I wouldn’t worry about it. People seem to worry a lot about what their receiver was sold as, and it’s due to a common belief that the 4473 “registers” the firearm and therefore a receiver marked as a “rifle” on the 4473 is now a rifle. But that’s not how it works; the 4473 is a transfer document, not a registration document. You bought a new lower receiver from Aero Precision, therefore — according to both the US Supreme Court and the ATF — you can build that receiver as either a pistol or a rifle. Also according to the same authorities, if you build that receiver as a pistol first you can always go back and forth between building it as a rifle or pistol, but if you build it as a rifle first then it can only be built as a rifle. Spending time worrying that you’re going to get falsely accused of a crime because of an extraordinary convergence of mistakes between the manufacturer, distributor (if applicable), dealer, and salesperson is kind of like spending time worrying that you’re going to get arrested for a bank robbery that someone else committed.