If you buy the firearm as a handgun, it is legal to convert to a rifle, shotgun or muzzleloader, as log as the barrel length is greater than 16". What determines the classification of the weapon is the serial number (how it was registered). If it was registered as a rifle, shotgun or muzzleloader, it can't legally be converted to a handgun. I know it's stupid but, that's it.
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It takes 43 muscles to frown and 17 to smile, but only 3 for proper trigger squeeze.