I have an old Herters press that will load 2-3/4 and 3 but it will not load 3-1/2. Most MEC were 2-3/4 only as I remember but that was before the 3-3/4 shell came out. Keep what you have until you have proved the new one. Then get rid of the old. Or return the 3-1/2 as not useable.