Is this an illegal stock?

[Adog5]

I want to put this stock (http://www.topgunsupply.com/parts-gsg-5-accessories/) on my GSG rifle, but it says that it would make it an SBR:

*NOTE* Mounting this stock on the GSG-5P pistol would make it a Short Barrel Rifle, which is illegal without the proper paperwork and ATF approval. Please contact your Class III FFL Dealer or ATF for more information.

I don't see the difference between this stock and a folding stock. Is this really illegal for CA because it would make it an SBR? I thought SBR have to do with the barrels <16". I just want to make sure this website doesn't know what they are talking about.

[Shane916]

The stock is legal to put on your GSG-5 rifle.

AFAIK no one in CA owns a GSG-5P

[69Mach1]

The GSG-5P is the pistol version. Putting a stock on that would make it an SBR. Just the standard NFA warning.

[oaklander]

Don't put it on the GSG PISTOL. The GSG PISTOL has a short barrel. . .

I want to put this stock (http://www.topgunsupply.com/parts-gsg-5-accessories/) on my GSG rifle, but it says that it would make it an SBR:

*NOTE* Mounting this stock on the GSG-5P pistol would make it a Short Barrel Rifle, which is illegal without the proper paperwork and ATF approval. Please contact your Class III FFL Dealer or ATF for more information.

I don't see the difference between this stock and a folding stock. Is this really illegal for CA because it would make it an SBR? I thought SBR have to do with the barrels <16". I just want to make sure this website doesn't know what they are talking about.