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You know what could let you use a pistolgrip without making a 'Shockwave' style of shotgun too short? An 'arm brace.'
The arm brace is a part which is intended to strap the form of your dominant arm to an extension on the rear of the gun, so that you can more easily fire a large handgun one-handed. This is completely legal, and the pretext is that it would allow disabled people, such as veterans lacking a second hand/arm, to fire bigger handguns. What these things can also be used as is stocks, and as long as you don't modify the brace to make it more stock-like (such as by putting on a recoil pad or something), this is 100% legal to do.
Buying something like an AR15 pistol, or AK pistol, and putting a stock on it, would mean that you're creating a Short-Barreled Rifle (as the barrel is shorter than 16"), but a brace is not a stock, legally, thus you can do this and it's fine. The same applies for these 'Shockwave' style shotguns, the brace is not a stock, and the overall length is 26", so the shotgun remains a 'Firearm.'