Based on the original old school benchrest definition of jam, the word is in wide misuse. Originally jam was, and is for me still, the maximum length that a bullet can be seated to without being pushed back as the round is chambered. Of course this varies with how much friction there is between the bullet and the inside of the case neck, which varies according to a number of factors: how much the neck is sized under bullet diameter, the hardness and thickness of the neck, and a couple of other things. If you are trying to find out this old school jam, neck tension matters. The tighter the case grips the bullet, the farther that the bullet can be forced into the rifling. I have a question for you. Of what use would the jam length be for a neck tension that you were not going to use for your loads?
