Even though you decided not to do it, let me answer some of your questions anyway. There might be other people who are interested.
The first thing you would need to do is to complete a Form 1: Application to Make and Register a Firearm. You can find that form here:
http://www.titleii.com/pdf/010205-Form1.pdf
You can fill out the form on-line and then print out TWO neat, completed Form 1s. As far as the ATF is concerned, you are manufacturing a firearm although in this case, all you are doing is installing a barrel. Since you are the manufacturer, you will need to engrave your name and location on the receiver of the gun. There are specifications as to this engraving.
After you have printed out your Form 1s, there are several steps you need to take before you can send it in. First, you need to get a couple passport photos. IMO, the easiest way to do this is to go to a chain drug store and have them take your picture and then process a whole sheet of small photos. Glue one photo on each of the two Form 1s.
Next you need to get fingerprinted. Make sure you use the correct fingerprint cards. Here in Las Vegas, the police department uses the correct cards already. You need two sets.
Then you get the chief law enforcement officer signature. He is signing that there are no local laws prohibiting you from owning this weapon.
Once the form 1s are completely filled out, pictures attached, and CLEO signature obtained you need to enclose the $200 tax. How you provide this money is up to you. I personally send a bank check so there can be no problems with them getting the money. I always envision that the day before they cash a check, I have some kind of personal crisis that drains my bank account and ATF sends back my paperwork with a letter that they couldn't cash the check: so, I send a bank check. Other people send a personal check so they can see that it was cashed and when it was cashed.
You will also need to include a form that testifies to the fact that you are a citizen.
You mail all this off to ATF and in a minimum of three months and a maximum of infinity, you will receive back one of the very Form 1s that you sent them, only it will now have a $200 tax stamp on the upper left corner. The tax stamp looks like a slightly large postage stamp and is marked as being valued at $200.
NOW, you can buy the barrel and install it. You can not legally own the barrel prior to this point in the proceedure.
You will hear a lot of people on the internet talking about jumping through hoops etc. And, I guess that reading about all those steps sounds like a big deal. It really isn't. That whole proceedure of making out the Form 1s takes me about two-three hours including drive time from home to the drug store to the sheriff's office to the bank. The only real PITA is the wait to get it back.
Someone will now chime in and say something like: "It's easy for you to say, our sheriff won't sign". Ok, that is another issue all together. The mechanics of doing this are easy. If you can't get the signature that changes everything. Here where I live, there is nothing to it. You drop the paperwork off and they call you when it is signed.
Whether or not this is worth it to you is something only you can answer. For me, it is.
