These requirements may vary for different jurisdictions.

Examples of minimum length requirements in federal law [ATF-NFA] include: 16 inch barrel length for rifles, 18 inch barrel length for shotguns, and 26 inch overall length for both rifles and shotguns.Feb 6, 2018

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More info.. short barreled pistol vs rifle..

AR-15 Pistol or SBR (Short Barrel Rifle): What’s Best For You?
Not sure to go with an AR-15 pistol and brace, or go with a tax stamp and SBR? We cover the pros/cons, current legal standings, and our top picks.

https://www.pewpewtactical.com/ar15-pistol-sbr/