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I’ve been thinkin about putting an 8.5 or 10inch barrel on my AR 9mm pistol. I have a 4” on it right now. My question is, I can put any length of a barrel on it that I want AS LONG AS I don’t exceed 16 inches and it will still be considered a pistol correct?

You can look over this for ATF's clarification on the issue: https://www.atf.gov/file/55526/download

Basically, you can do whatever you want within reason assuming your lower did not come as a long gun from the factory. You can even convert it to a rifle and back again. Just make sure that if the overall barrel length is < 16", you don't have a stock or forward grip etc on it to make it an SBR.

Of course, IANAL

Where do you see a maximum barrel length for a pistol? There's a minimum barrel length for a rifle without SBR stamp, but I can't find any maximum legal barrel length for a pistol. I'm also pretty sure if there's no stock and the OAL is more than 26" then a vertical forward grip doesn't make it a NFA AOW.

You're fine to put whatever barrel length upper you want on your pistol lower.

Thanks. Much appreciated!