Don't forget about the sales programs run by the NRA, the NBPCRP, the DCM and the CMP - all legitimately selling US Property rifles and pistols of all types and eras - including Trapdoors.
Plus, if you knew how, and were willing to do 'all' of the requisite paperwork - you could buy from Ordnance, but it was a major pain, unless you were walked through the process.
That saw stuff like Winchester Model 70s come out of the woodwork, as well as other things of interest that Uncle Sam bought and decided against, for one reason or another.
After the GCA of 1968, it got harder, but if you were military - it was still a do-able thing.
My personal favorite stories involve the WWII bring-backs of the M1 Carbines.
You know - the ones with all five (count 'em, five) Modification Work Orders) that didn't happen until the Postwar rebuild.
Taking home a WWII Carbine 'at that time' was the equivalent of taking home a Norden Bombsight.
To say it was frowned upon is an understatement...
A lot of US-issue stuff came home from Vietnam, too - all from Marvin the ARVN and PHILCAG and any other US-supplied 'allies' we'd had at the time - all 'our' stuff could be had for a price, since they could get more and more and more - but the big problem was in getting it home - a problem often surmounted by 'knowing a guy' who flew for a living...
Did purloined stuff come home in dufflebags and seabags?
Good Lord, yes, - GIs, Zoomies, Swabbies and Jarheads are notoriously opportunistic thieves, as all COs and MPs know - but after a war, no one cared all that much.
Later, we got a tighter handle on things - and once the Automated Property Book came into being, a lot of pilferage just stopped, because it was too easy to track the serial numbered gear and sensitive items regularly, and not make changes, as could be done with the old Manual Property Books.