

He said he had no docs for it. I asked about F4 status, he said he didn't have one and if I bought it he would just "forget about the sale"

If it is complete, it is a DD and should be registered. Tell him you will buy it but without the barrel but you know someone who wants a barrel. Have him pull the barrel. sell you everything but the barrel. Have a friend buy the barrel. Once you get the Lahti, register it on a form 1 as a DD build. Once approved go get the barrel from your friend, install it and then you have a registered legal Lahti. A lot of people don't like this but it is legal on your part, the seller is currently in possession of an illegal DD.

If it is complete, it is a DD and should be registered. Tell him you will buy it but without the barrel but you know someone who wants a barrel. Have him pull the barrel. sell you everything but the barrel. Have a friend buy the barrel. Once you get the Lahti, register it on a form 1 as a DD build. Once approved go get the barrel from your friend, install it and then you have a registered legal Lahti. A lot of people don't like this but it is legal on your part, the seller is currently in possession of an illegal DD.

I believe some were converted to 50 cal so there would be no form 4. Otherwise…. Something doesn’t sound right. Need more details
https://www.proxibid.com/Firearms-M...0-cal-Anti-Tank-Rifle/lotInformation/32355742

Yes, many were converted to 50 BMG by the owners. Many years back, Sam Cummings of Interarms fame, the original importer of the 999 20mm Lahtis, had around 250-300 converted to 50 BMG by Charles Erb. He was upset that his imported Lahtis were being used for criminal activity.
Back in 1965 Joel Singer and his gang bought two Lahtis from Interarms and had them shipped to upstate NY. He had a feeling that they were being watched so he broke into the shipping company over the weekend and took his guns, then went on to rob a Brinks location by shooting a hole through the vault wall. They got $430,000 (about $3.3 mil today) and Singer did get caught. This was the basis for the Clint Eastwood movie "Thunderbolt and Lightfoot".
Around the same time, the fBi came across another gang that was going to do the same type of robbery, but they were stopped.
Looking up these robbers made for a good read. Is it possible to reload these shells? I would think you'd need a whole scaled up loader and hardware.