They're not the same thing. And remember, the ATF approved slidefire stocks (as long as they don't use springs to push the gun back forward). A good lot of firearm legislations in the US don't make sense and shouldn't exist, like import restrictions. The same model of gun could be illegal to import but be legal to own and manufacture in the US, it's stupid. Barrel length restrictions, same thing. And I think those stupid legislations are part of the reason why the pro-guns don't want to give even an inch to the anti-guns.