The thing that limits how far a bullet may protrude from the case neck of a loaded round (if it is limited) is the constriction that starts where the lands begin ahead of that portion of the chamber designed to house the case neck. In the case of many target style chambers the ends of the lands are chamfered from zero to full height at an angle of one and one half degrees in relation to the centerline of the bore. If a bullet just makes contact with the angled beginnings of the lands, without being significantly marked we say that it is "touching". With any loaded cartridge length ,(with that particular bullet), that is shorter than one loaded so that the bullet just touches the beginning of the rifling will have the bullet at some distance back from the point at which it first makes contact with the angled beginning of the lands. This is the distance off of the lands, also referred to as jump. Conversely when a bullet is loaded to a longer cartridge length (than touch) it is said to be into the rifling by the amount longer than a round loaded to just touch. Another term commonly used is jam. This is the maximum length that a given bullet, at a given neck tension can be loaded to without the bullet being set back in the case neck when the bolt is closed. Many believe that referencing loaded bullet position as so much off of (less than)jam is more conveinent than referencing the touching length, primarily because all that is required to determine this lenght is to load a round (or dummy round) long and then chamber it, in the process pushing the bullet back to jam length. (I know that this is overly wordy, but I believe that it is correct and I am too tired to clean it up... maybe tomorrow.)