This might help some
www.serbu.com/nfainfo.htmThere was another cool link that I can't find
. It had all the ATF forms in a downloadable format. You might call ATF's forms department and ask them for the forms. They will mail them to you.
Make dang sure you get all the forms filled out and approved before you start! And, yes, the Sherif or police chief, whichever you have will have to approve of you doing this and sign the form, before it is submitted to ATF. So how well do you know the sherif or what do you know of him plays in heavily here.
As far as putting a choke in the barrel, the most common method is to ream it, then use a tap to thread it for the particular brand of screw-in choke tubes you plan to use. It can be done by hand, or with a lathe. If everything has the proper pilot, and you are slow, patient, and careful it can be done by hand. Check Brownell's prices first as the tools do get expensive.
There are also sleaves that can be soldered in to give you a choke. There are other methods, like the Tula or jug choke. I might recommend a gunsmithing course somewhere in this reply, like Thomson in PA.
I have toyed with similar ideas off and on, but usually talk myself out of it. I was even thinking of getting a "manufacturer's"-style FFL. The problem with these licenses, and the NFA stuff if I understand correctly, is that if you move, you have to refile or fill out more forms. You must also investigate if the place you are moving to allows such things or activities, plus get the new LE official to sign your paperwork, if you can.
The moral of the story is investigate the paperwork and see if the chief is friendly or not first.
The other option is to keep an eye on this thread for news
ar15.com/forums/topic.html?b=6&f=1&t=194263&page=1Ya, I know, it's an 18" barrel, but dang it's cool!