Well when I was a kid the law in the US for deactivating (DeWat) war weapons was that the breech had to be welded shut and the barrel and guts filled with melted lead. This let folks buy Tommy Guns, Schmeissers, Maxim tripod guns, cannons and all sorts of WW2 goodies through the mail from the back of magazines.
Problem was some of the welds were quickly done and more than one person found out you could smack the weld with a cold chisel and often it would break open. Regarding the filled barrels, well issue one was a loose barrel could be sold intact, so if you could get the lead filled barrel out, just put in a new one. If not (and this worked for receivers that had lead in them too), the melting point of the steel was much higher than that of the lead. So you hit the gun with a torch the lead melts and you just pour it out. Yeah sometimes small parts got ruined. That's okay, the parts you needed were for sale almost everywhere. Truth is, although it broke every law on the books, when I was a boy I knew several people who had full automatic supposedly DeWat'd machine guns. Amazingly the laws on explosives were even laxer. You could buy dynamite at the hardware store, Amatol too. You couldn't ship live hand grenades or mortar shells in the mail, but you could by commercial carrier's such as REA or UPS.
Then came the 1960s with assassinations and Vietnam War protestors and bombs in department store bathrooms and the ilk. In 1968 (rightly or wrongly) the US Congress changed all those laws.
The old DeWatt was no longer considered any different than being fully functional. It either became licensed or it was confiscated, or it was illegally hidden (often with the forlorn expectation that the law would be changed back). We experienced some mutilations as pistols with shoulder stocks were believed by the politicians and Hollywood to be super accurate murder weapons (see Day of the Jackal and From Russia With Love), so to stay legal for a long time old Lugers, Brownings, Mauser and other pistols designed to take a shoulder stock had the stock attachment either ground off or filled with weld metal.
The US Federal Agency charged with enforcing the law (BATF) set up guidelines for each specific weapons type showing exactly where they had to be cut to no longer be considered a weapon. Generally each receiver is cut into 3 different pieces. Even then they ran into problems when US Military surplus firearms were so cut and sold as scrap metal. Some folks would find 3 parts, build a jig and weld the 3 parts back into one, then fix the gun so it would shoot, then sell it or use it in a crime. Of course they didn't tell the buyers how they got the gun. So sometimes those weapons would explode in the consumer's face. The end result was the scrapped guns were now also sqooshed in a press before being sold as scrap metal. Congress changed laws a few more times. In general now a machine gun can not possess it's receiver. A lot of guns that were banned are now being sold without steel receivers, which of course gives you just so many gun parts without the gun part. Some vendors also sell 'looks like the real thing' receivers made of zinc or another pot metal. Up until the 1980s you could buy one of those, then by paying exorbitant license fees have someone make you a real receiver and then merge that with the parts kit to make a working gun. This clear threat so terrified Hollywood and the anti-gunners that yet another law was passed prohibiting anyone but government agencies from requesting a machine gun's frame be fabricated. So now, if the gun is licensed it can be sold and possessed, but no new functional machine guns can be made or imported. Except on govt. order of course. Lots of gun kits are out there now with almost complete guns, but no frame/receiver.
Are there still any DeWatts? Yes, of course, even though they are no longer being made as DeWatts. Those pre-1968 DeWatts that were never restored to function are licensed as if they were functional and many still sit in museums and similar places with no one but the owner and the govt. knowing they are DeWatt. Generally though if you own a DeWatt, sooner or later you will sell it to a licensee who will convert it to full function then sell it at a huge profit. Likewise, cannons are usually considered legal to own if the breech mechanism is missing. Further some towns and cities have their own definition of locally permissible DeWatts to possess in your home in certain neighborhoods. Plugged up shotguns and stuff like you wrote about. Local rules.
Because of all this fully functional and licensed military type weapons sell for small fortunes. Many of the now rarer types have been known to go for hundreds of thousands of dollars at auctions. Kind of a good investment if you bought it out of the back of a magazine for $30 back in 1964 then registered it in 1968.