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If you do not violate either one of the length measurements it is legal. The act of cutting the stock does not mean anything is violated, it is though when the said cutting brings it to under 26".

From the ATF website, here is an example of one that is not legal.

l2ga. Crude Manufacture

Classification

Image of a 12ga Crude Manufacture
Weapon Made from a Shotgun

Distinctive Characteristics

Barrel less than 18 inches and/or overall length less than 26 inches, stock altered, barrel cut down. Often mistakenly called a "sawed off shotgun." In this instance overall length is determining factor. However, if the overall length is 26 inches or more and barrel less than 18 inches, this weapon would still be classified as a firearm under the Act.

Rate of Transfer Tax

$200.00