Many of these are legal, you just have to declare them as such. For example, you can not saw of a shotgun in your own. But there are several short shotguns in the market.

Also, I really want to know in what world you can classify a short barreled rifle as a pistol, and not have an officer laugh at your form before denying the classification and registration. Or classifying a sawn off shotgun as a “firearm”