Correct information!!!!
A Mossberg Shockwave, Remington Tac 13 and Tac 14, along with other makes of smoothbore firearms are legal because they were made with virgin receivers that have never had a shoulder stock installed and are NOT designed to be fired from the shoulder.
The legal definition of a shotgun is: a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger.
There for it is illegal to make a Shockwave from a shotgun because it started out as a shotgun designed to be fired from the shoulder.
The reason a standard pistol grip and 18" barrel is fine to swap out with a stock is because the firearm is still over 26" in overall length AND has a barrel at least 18" in length
One can make a Shockwave out of a standard Model 500 only if they submit a NFA Form 1 along with paying the $200 tax and only after the form has been approved. It is way cheaper and easier to just go buy a Shockwave. No need for NFA paperwork and registering a shotgun as a SBS or AOW.