I think there might be another thread on this but hell I'll join:
Standard Mossberg 500
Colt .45 1911 ACP
I built an 80% AR-15 that doesn't need to be registered with the Fed
For those that don't know - you can purchase an 80% completed lower receiver, you don't need to register it, can pay cash, and receive it at the time of purchase.
You can machine this "yourself" aka find a buddy with a CNC Machine and "press the button" OR you can literally do it yourself.
Once this 80% lower has been machined, you have a fully functioning lower that you can get a lower kit for - throw in an upgraded trigger and stock if you'd like.
All you need to do now is purchase an upper receiver - once again you can pay cash, receive it the same day, and it doesn't need to be registered.
Put the upper and lower together and you've got yourself a fully functional AR-15 that no one knows about.
I live in CA, and it's even legal here, of course I get my 30 round mags across state lines
(I NEVER take them outside my house of course)
Throw some optics, grips, other tac stuff, and you've got a badass rifle you built yourself on the cheap.
Excluding optional equipment, the basic rifle I built cost me around $600-700, and you get a basic education in rifle building along the way, which is priceless.