At the risk of sounding like a wise a#$, because that is the way PO Ackley designed it! Your existing chamber can be rechambered "as is" but, if you want to shorten the neck length the barrel would need to be set back which is a real pia on a lever rifle. Another factor to consider is that by shortening the neck, I am going to assume you plan on moving the shoulder forward to gain more powder capacity(if not there is no reason to mess with the neck length) I would be concerned about potential feeding issues. Also a shortened neck would decrease the "hold" on the bullet in the magazine, under recoil, and that could result in more problems. Sorry to sound so negative, but it seems like if it can go wrong it will happens all to often.