I think there are two possible culprits. 40 year old oil that turned to glue or if the gun was not oiled at all, galling of the aluminum cart to the steel chamber walls. I had to actually cut apart an old 78G that had been left sitting in a cellar for a few decades. The steel tube in the barrel housing had galled to the aluminum housing. If you have worked on the 78G you know what that tube is.