No way he made a gun that way
The way he made his short stroke, lightweight carbine in the movie was highly implausible. He would have needed machine tools to make the receiver, barrel and most of the other parts. (Accuracy as shown would have been impossible without a rifled barrel.) Where did he get the springs? He must have been working in a well-equipped machine shop. He was mentioned in an article I read, but they didn't address this issue. But hey it was a movie, based on fact, but still for entertainment.
Remember Rabbit Ears with tin foil?