I don't have any NFA items because I refuse to play the registration game, although there is a fair amount of relief for that if you create a trust (a legal entity) and the trust owns the NFA items. Everything in this post is the product of my reading and indirect experience with friends engaged in NFA purchases and ownership, and is not the result of direct experience, and certainly is not a legal opinion.
Every SBR you create from a pistol has its own unique serial number. You essentially become the manufacturer if you add the parts to convert the pistol into an SBR. It can be as simple as one of those Glock conversion kits that snaps together and can be readily switched back and forth, or as complicated as altering a pistol receiver to add a butt stock. Folding butt stocks are popular, as they retain most of the pistol's compactness while doubling the accuracy with a butt stock.
You need a local law enforcement sign off, and the Fayette County sheriff, Kathy Witt, does not do that. Most Kentucky sheriffs have a "shall issue" attitude, a few have a "may issue" attitude for friends and people they know to be upstanding citizens, but Sheriff Witt has a "do not issue" policy. The previously mentioned trust can work around that problem.
Yes, it's $200 and a fairly short but confusing BATFE tax form. Unless you go the trust route, I think they get a copy of your fingerprints. Expect 3-6 months for the BATFE to do the background check and process the paperwork. Sure, they should be able to do it in a day, and a customer oriented private company would do it in a day or less, but the BATFE has no incentive to be efficient about it. I think they do this on purpose, much like a waiting period to buy a handgun in many states.
If you manufacture an SBR, you need to get permission from the federal government to transport it across state lines.
You need another $200, tax form, and lengthy wait for each suppressor or SBR. The suppressor may be used on any weapon. The suppressor is registered, not a combination of suppressor and a particular firearm.
Forming a trust is a good way to loan an NFA item to someone else. You simply add them to the trust. If you're found in possession of someone's NFA item and it's registered to them and not a trust with your name on it... bad juju. Probably one of those "ten years in prison" things. Putting someone else on the trust probably complicates individual ownership issues if the trust ever needs to be revoked.
In addition to suppressors and short barrel rifles (SBRs), there are also short barrel shotguns (SBS) and "any other weapons" (AOW). If the regulations allow a weapon to be registered as either an SBR or AOW, you're generally better making it an SBR. That way, there's less confusion about the legality of adding a vertical foregrip (VFG), which is OK on a rifle but not a pistol and maybe not with an AOW?
SBR = rifle with a barrel less than 16" or an overall length less than 26".
SBS = shotgun with a barrel less than 18" or an overall length less than 26". Often called a "sawed off shotgun".
All of this is from memory, and I AM NOT AN EXPERT, and do not even own any NFA weapons. Personally, I think the National Firearms Act of 1934 and the Gun Control Act of 1968 are BS.
It's difficult to get a straight answer out of the BATFE, and when you do, get it in writing, because you may need to prove that the BATFE said something later if they haul you into court. The laws are arbitrarily enforced, and that's even more true of the BATFE rules. The rules are bureaucratic decrees, created by people who were never elected to any legislative position, yet they're enforced as laws.
Someone needs to make an easily searched Frequently Asked Questions page or info web page that is essentially NFA For Dummies. I've never seen a link to such a page, and there is a lot of confusion and misinformation on the internet on this topic. The BATFE are the obvious choice, but I think they prefer all of the confusion. It helps prevent the widespread use of NFA items. Only the intrepid will fight their way through the fog of misinformation and register an NFA item when the penalty for getting it wrong is 10 years in jail. Others are probably afraid to make such an info page for fear of misinterpreting some vague BATFE rule or law, or drawing the attention of the BATFE for simplifying a process that they prefer to be complicated.
As gutshot mentioned, subguns.com is a good resource. It's not designed for newbies, but I've read many stories of people getting a good answer there that helped them stay out of legal trouble. Hey, I just looked on subguns.com, and found a good page with links to help navigate the mine field of gun laws and understand them.
http://www.subguns.com/laws/laws.htm
The subguns.com page linked to an NFA FAQ, so I guess it does exist!
http://www.titleii.com/bardwell/nfa_faqhtml.html
Obviously, anything you read there is way more trustworthy than anything I've written here.
In my world, there would be no NFA or GCA, and anybody could buy a good suppressor at WalMart for $50.