You can build it so long as it's legal. First you need to check state laws because if you are in a state like mine there are many restrictions on what you can have IE the Assault Weapon Ban :barf:. Next you need to be sure that what you are building dosen't violate the Nation Firearms Act. This means that you can't build things like Machine Guns, Suppressors, Short Barreled Rifles / Shotguns, Destructive Devices, and AOWs (a smooth bore pistol is considered an AOW). You actually can but you need to get your tax stamp first (BTW you can't get one anymore for a Machine Gun). Also you need to be careful if you are using imported parts since building an "Assault Weapon" using imported parts is illegal as well.
I have mostly non-sporting firearms
In NJ, technically speaking, ALL guns are illegal
Also in my state there is such thing as a Class III BB gun :barf:
Happy to say that despite the NJ AWB I still manage to make my guns look scary