Someone buys an AR rifle for whatever reason they disassemble it and decide to sell the stripped lower to someone else.
They transfer it through an FFL and the 4473 is marked receiver for that transfer can that receiver be used for a pistol build?
Along the same lines someone has a Remington 700 and the stock gets broken. Person sells the barreled action transfers it through an FFL. FFL marks the 4473 as a barreled receiver can that barrel be cut down or replaced with a shorter barrel and built as a pistol?

Per the letter of the law… once a rifle always a rifle.

But if you someone was the second or third owner of a used stripped receiver I don’t know who the responsibility would fall on to prove what it was born as.

I know I’ve bought a couple used lowers and have zero idea what they started as.

FFL will receive the lower or receiver and log it in as it is handed to him/her

so you sell your stripped lower to the FFL, or the FFL does a transfer for you they will log it in as a lower, and out to the next buyer if it is sold as a lower,

matters not what it was the day before it goes in that dealers books,

ditto pistol frames, or bolt action receivers, etc

The FFL does not have the ANY authority to decide if the lower or reciever was or was not originally a rifle. They make mistakes and were not chartered by Congress to make the determination according to law.

If there is a question over it the ATF will revert to the manufacturers declaration of what it was shipped as, first, then if there is a track to determine what subsequent owners assembled it for use, any one of them may be their final determination.

I say that because there are more than a few online who state "I always build it as a pistol first" so they can legally swap it back and forth. There had better be evidence level documentation, not "but I say so" and a willingness to stand in court paying the lawyers fees to do it - the ATF's enforcement mechanism isnt a guilty finding, its financial punishment. They will treat you like Kyle Rittenhouse.

If by a long train of abuses and usurpations there is a point where the judge believes it became a rifle first, poof, it's done right there. Expect to hear "ignorance of the law is no excuse." Unless it can be substantially proven with documentation and evidence, if it could be a rifle lower or receiver, it's just easier to pass on those and obtain one noted by the manufacturer as never having been assembled. Them > you = proof in court. The cost of an AR lower isnt really much of the total build - why bother accepting the risk? Anything in the way of a bolt action reciever? Only with the papers from the factory stating it was a pistol or unassembled first. No word of mouth, hearsay, etc. Serial number confirmation or no go.

Playing around in the gray areas is exactly where the ATF wants you to go - because they make the rules, they INTERPRET the rules, and they enforce the rules. It's a one sided game and we shouldn't have ever let them into it - but we elected Congressmen in the 1930's who stabbed us in the back. Even then it was a rigged game.

The burden remains on the government to prove the act and intent. If you purchase a stripped receiver (or a pistol receiver or whatever else), build it into a pistol, and then somehow it is determined that at some point in the past it was a rifle, that will not result in a criminal conviction.

It's no different than if you bought a used gun and later found out it was stolen. You may lose it, but there is a requirement that it is knowingly and intentionally possessed.

Once a rifle always a rifle……

Well kind of.

If you bought a stripped lower & built it into a rifle then decided to sell it face to face (where no 4473 was required) , that individual can make it into a pistol if they want as the original lowers serial number left the factory as a stripped lower not a complete firearm as in the OPs original post / question.

But if you build a stripped lower into a rifle then sold it and it went through a FFL where a 4473 had to be filled out at that point it’s a “rifle for life”.