As a rule you can make a pistol bigger with little danger of being outside the law. Except it is illegal to add a shoulder stock to a pistol, unless it's a luger or one of the others specified on the BATFE list of exceptions. If the pistols barrel is 16 inches it is acceptable.. If the overall length of the pistol is 26 inches it is already a rifle, adding a stock would have no effect. If the overall length is less than 26 inches I'd speak with BATFE BEFORE altering the pistol..
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