Admittedly I am not real familiar with semi auto Thompsons. This one is not for sale but the owner just got it from his father's estate. On the phone the conversation was all about nfa weapons. But I was shown it today. He tells me it fires from an open bolt. All the innards are WH27a1 and I have not shot it. Would like to take it out to the range out of curiosity. The bolt locks back and I am sure a drum can be loaded without a 3rd hand. It is not an sbr. With the bolt forward, I pull the trigger and the hammer releases. The owner says he recalls as a kid it ran like a full auto, could that mean from an open bolt, he thinks but that was 30 plus years ago. What am I missing? Oh, with the housing separated, it functions like a semi meaning the sear catches the bolt.