To be an assault weapon under Federal law, it would first have to be a semiautomatic shotgun, which is defined as a repeating firearm which utilizes portion of the energy of firing a cartridge to extract the fired cartridge, and chamber the next round. So, as already stated, a pump shotgun (or double barrel) could not be considered a semiautomatic assault weapon.
However, if you do have a semiautomatic shotgun, and are considering putting a pistol grip on it, you do need to be careful. If any two of the following features are on the shotgun, then you have created a semiautomatic assault weapon, and it is controlled by Federal law, and almost certainly State law:
1. A folding or telescopic stock;
2. A pistol grip protruding conspicusouly beneath the action of the weapon;
3. A fixed magazine with the capacity to accept five rounds; or
4. Ability to accept a detachable magazine.
So, if your semiautomatic is like most, its chamber will hold accept five rounds, with no plug in of course. Im not positive, but I suspect this would be defined as a "fixed" magazine for purposes of the Gun Control Act. Thus, that "fixed" magazine combined with the addition of a pistol grip protruding conscipuosly beneath the action of the weapon is likely going to be something that is controlled by Federal Law, and illegal to have without having completed and applied for same.
I also would of course recommend that you check with your local and State laws. Almost all states mirror the Federal Gun Control and and National Firearms Act, or have less stringent requirements. Only a handful restrict in excess of the Federal GCA and NFA, but you might live in one of those.
Nim
