Getting The Parts

If you live in the United States, your sources for parts are many and easy to obtain...just pick up a recent copy of the Shotgun News and you'll find many vendors who supply complete kits. Some supply the whole works (but you will need to go through your local FFL dealer to get these); others supply complete kits less the lower receiver (which is the only part requiring an FFL) and they will ship directly to your door. You'll still need to come up with a lower receiver, either through an FFL dealer, or by buying a receiver casting and finishing it yourself. This last route requires no FFL, and is much more challenging and satisfying when the project is complete.

I live in Canada, so the challenges are much more magnified for me, due to the nature of our gun laws up here. Now I say challenging, but not impossible to overcome. (And for the record, I finished, and registered my AR-15 more than two months ago, in case anyone is listening or interested.) The laws on both sides of the border require that you arrange to purchase all of the major components (like the barrel, bolt assembly, etc.) in the US, and then import them yourself PERSONALLY into Canada. Since only a finished receiver is considered to be a firearm, importation of the parts (less receiver, of course) is perfectly acceptable. While the Customs guys are concerned that you may be building illegal weapons, they really can't stop you, but be reasonable...show them your firearms license. And remember, an AR-15 is a restricted weapon up here. You must not only be licensed from firearms Acquisition, but it must also be for Restricted class, as well. This applies to the receiver and the finished rifle. If you want to know the steps I went through to "stay legal", they are:

Import the receiver casting and parts kit. The receiver casting is just that...a chunk of metal, so no restrictions apply. I talked about the parts kit above;
Finish the receiver, including permanent stamping of a unique serial number;
Contact the good people at the Canadian Firearms Centre and advise them what you are doing, and what stage in the production you are at;
They wil send you a form (676E) to fill out for registration. You are the manufacturer;
You will then need to contact the Restricted permits section, who will
Issue a permit to transport your new receiver or rifle* to a verifier (who you will find locally before you call);
Get it verified and signed off; and
Mail in the application to register your new rifle.

I sent mine registered mail to ensure it got there. Once it's in their hands, you're done everything required to comply with the laws.

* In my case, I gave them the option: register the receiver first, then complete the rifle and then modify the registration, or do it all at once. They kindly agreed that one step was far more efficient for both of us and we did it all at once.

The Project
The Worklist
Finishing Up