Can't be purchased w/ LEO credentials, has to be an ATF registered SBR which anyone can do for $200. LEO status change makes no difference...
Well I can promise you he didn't pay the $200 ATF fee. He's also not the first LEO to be able to get his hands on Class III Firearms alot easier than we can.
Actually anyone with a credit card can order a short barrel and/or upper. No credentials/paperwork/stamp needed.
However if you want to actually attach that short bbl to a lower it better be registered first. Beyond that, there is nothing stopping anyone at all from buying a short barrel. My 9.25 inch upper was shipped USPS straight to my cubicle with no ID, no signature, no nothing.
Can you get a short barrel upper and attach it to a registered pistol lower without a tax stamp?
I would think so, as long as it doesn't have a shoulder stock or FUG attached...