I can offer some general information.
The Mossberg 590 Shockwave is a "firearm" as defined by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) in the United States. It features a unique design with a 14-inch barrel and a Raptor pistol grip, designed to be held by the shooter's hand and not shouldered like a traditional shotgun.
Modifying firearms, including the installation of accessories such as pistol grips, can have legal implications and may be subject to specific regulations depending on your jurisdiction. In the United States, the ATF has specific regulations regarding the classification of firearms, particularly those with short barrels.
It's crucial to consult and follow the laws and regulations of your local jurisdiction and consider seeking guidance from qualified professionals, such as firearms dealers, attorneys, or knowledgeable individuals who are well-versed in local firearms laws, before making any modifications to a firearm. They will be able to provide you with accurate and up-to-date information based on the laws in your area.