Converting AR pistol to carbine?

[clearconscience]

So from what I understand if you have a AR pistol you can convert it to a rifle, but you can't switch a rifle designated lower to a pistol?

Is this correct? Or does it have to stay whatever the lower is designated as in that platform?

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[Nwcid]

You are correct.

There is no such thing as a "designated" lower though.

If the lower was originally built in a configuration meeting the definition of a pistol then it will forever be such (but you can legally do other things with it). If the lower was originally built into a configuration meeting the definition of a rifle it can only ever legally be a rifle or NFA to a SBR.