Ok, here's how it works:
After prohibition was ended, the newly created ATF needed something to do with themselves, that is, something they could actually do in public, not just alone in their bathroom, bedroom, or closet. With Tobacco and Firearms left, and as much as people smoked back then, firearms were the next available way to occupy themselves. Plus, prohibition had spawned a rise in organized crime, which had managed to obtain weapons that gave police a run for their money.
So congress passed the NFA - National Firearms Act. It drew up three cookie cutter categories of Rifle, Pistol, and Shotgun. If your gun didn't fit these categories, there were other categories called NFA Weapons.
NFA weapon categories are:
Short barreled Rifle - has (or had) a stock, has a barrel under 16" or overall length under 26"
Short Barreled Shotgun - has (or had) a stock, has a barrel length under 18" or overall under 26"
AOW - Pistol with foregrip (forend is restricted under the AWB, not the NFA), handle not perpendicular to barrel (ie pen gun), smooth bore that had never had a buttstock, or anything else that defied the other categories.
Destructive Device - any projectile with an explosive charge of over 1/4 ounce, or any gun with a bore over 0.5" EXCEPT for sporting shotguns (and certain big game rifles)
Silencer - Yes, those
Machinegun - Anything that fires more than one shot per pull of trigger
To own an NFA weapon, you must send an APPLICATION (instead of being checked against a database and getting the benefit of the doubt unless there's trouble) and the application must be APPROVED. Yes it includes being signed by your local sherriff. This proccess can take over a year. And you must pay a $200 transfer tax each time (imagine a $200 transfer tax on a $5 silencer in 1935.) You may BUILD an NFA weapon by paying the same $200 transfer tax. However, to buy (not build) an AOW only has a $5 tax. AFTER you are approved and have paid the tax, THEN you may pick up your weapon.
For historical reference, remember that in 1935, the 1903 bolt action rifle was standard issue. The AK-47 was 14 years away from adoption by the Soviets, and the M1 Garand was right around the corner. The AR-15 wasn't even a twinkle in the juvenile mind of Eugene Stoner.
So to own an NFA weapon, you had to apply for approval, and have it registered in accordance with the NFA. Note that there is no official federal gun registry apart from this. Only records of NICSs which are supposed to be destroyed, which as far as I know do not identify the firearm being purchased (that info is kept by the dealer as required by law). It's all this crap for an NFA weapon or just buy a pistol, rifle, or shotgun from a hardware store with no paperwork.
Scoot ahead, pass up the Gun Control act of '68, and go to 1986. There was a bill read and approved by most of congress. Then someone snuck a little rider in there banning all new production of machineguns. Most of the people had already read it and had no idea it had been tampered with when they voted on it.
This means all machineguns that can be transferred between regular citizens had to have been made before 1986.
Meaning no more full auto Browning A5s, no more Vollmer conversions of HK-91s to G3, no more AR-15 auto sears, and no AKMs. So basically no new designs. Great way to kill the design of products that would make the armed services better off.
Violating this law is a friggin big deal. It is the most prosecuted firearm law. Especially the full auto part.
Think about it. No one will no you have a short barreled rifle, a flash hider, a folding stock, or whatever unless they see it. But a full auto you can hear for miles.
