Ok guys, I'm getting ticky on this one....
But
1) Making a "Short Barreled Rifle", a title II "NFA" weapon is not illegal. You have to pay a $200 tax though. Just like silencers, destructive devices, etc aren't illegal. You just have to pay a tax to make or transfer ownership. Making a short barreled rifle without paying the tax is illegal. Huge difference.
(note: this is at the federal level, SBRs may be illegal at the local level in some places)
2) Filing a Form 1 to make an SBR is not a "permit", it is paying a tax for the creation of this NFA item.
The Form 1 with the canceled stamp sent back to you is not a "permit", it is proof of tax paid on the firearm. Having a Form 1/4 with that stamp is nothing like having a concealed carry permit from your state. A CC permit/license is exactly that, a permit or license. A $200 series 1934 NFA tax stamp is proof that you paid a tax.
The reason I am ticky about this is because there's a lot of just plain incorrect info floating around out there about title II items, and if we are going to get these laws changed, we need to have a good understanding of what they are.
Personally I wouldn't own a stock for a pistol even with a 16" barrel without filing the form 1, it's just safer that way.