The stripped lower receiver is the registered part, ie "the gun." All other parts, including a complete upper receiver, can be shipped directly to your house. It is not hard to build a completed lower receiver from a stripped lower reciever, there are good instructions online, such as the AR15.com website. You will need some simple tools, especially some punches which won't mar the finish such as brass, maybe some needle nose pliers and a little tac hammer. Building a complete upper receiver is possible also, I'm told requires some more specialized tools, esp. to check for proper headspacing. I'd buy the complete upper and assemble or buy the complete lower and just snap them together for your new AR-15. Think about what you will use your gun for prior to assembly. If you plan a lot of target shooting or varminting, consider a 2 stage match trigger like the one sold by Rock River Arms. They sell this trigger in a kit with all the other lower parts you will need, bye the way. For a carbine, I like the Magpul CTR stock over the standard inexpensive 6 position stock. I'd go with milspec buffer tube over commercial as it increases your options significantly. Remember, you can buy another upper receiver and just slap it on your lower, so think over what you want in your lower before you build. Make sure to safety function test the lower after building to make sure the safety works and don't pull the trigger and drop the hammer all the way unimpeded without the upper attached.
