In the US, shotguns originally manufactured without shoulder stocks (and thus not legally shotguns), with a total length under 26 inches, are classified as an ''Any Other Weapon'' by the BATFE and have a $5 transfer tax, if they are manufactured by a maker possessing the appropriate Class 2 Special Occupational Taxpayer Federal Firearms License. In order to convert an existing shoulder-stocked shotgun to a short-barreled shotgun or an existing pistol-grip-only shotgun to an ''Any Other Weapon'', a private citizen must pay the standard $200 NFA tax.

is it legal to have a pistol grip on a shotgun?
yes