I am just trying to do some research on the subject and I have no plans to do it was just wondering the legal issues involved.
Go search for "NFA FAQ" by Bardwell for the best, if slightly dated, coverage of the issue.
No such thing.
To own any "NFA" items, you fill out a Form 4 (dealer will walk you thru it), or a Form 1 (if you're going to make it) and pay a $200 tax. That's a one-time thing, not a "permit". You must have BATFE approval BEFORE receiving/building the item.
I have seen Civilians with Select Fire Weapons that have been manufactered within the last 5 years, are these weapons illegal ?
Yes. Very.
The only "exception" is that the part that made it full-auto is considered the machinegun itself, and was registered as such before 1986.
I would like to own at some point in the future an Assault Rifle that is Select Fire
It will cost you over $15,000.
Find a seller, fill out a BATFE Form 4, get the local CLEO to sign it, attach passport-type photos, get fingerprints, attest to being a citizen, attach check for $200, mail to BATFE, wait a couple months for approval, get gun.
To "convert" something would require getting a registered DIAS or Lighting Link, a tiny part which legally
is the machinegun, and which will cost you almost as much as a full machinegun - IF you can find a legal seller.
Go hang out at subguns.com and silencertalk.com for a while for more nuances.
Upshot: it's gonna cost you. A lot.