I have a 45LC 1894 Marlin with an oversized chamber. I have been told the factory does this deliberately to aid feeding, and by using oversize reamers, they can cut more chambers before they are too small. Anyhow, I am starting to get an inordinate amount of case splits. I measured the diameter of a fired case and found it was .485. The maximun case diameter according to the Lyman manual is .480. My Lyman T/C sizer die sizes the cases to .473 diameter. There is 12 thousandths expansion and resizing here, and I think this is what's causing the splits. The loaded ammo rattles going into the chamber. Dummy rounds, using fired and unsized cases, chamber easily, but there is no neck tension on the bullets. They are locked in place by the crimp, but they spin. I am using .454 bullets. I feel less sizing and therefore less expansion will solve this problem, plus a larger case will align the bullet better to the bore, improving accuracy somewhat.
What is the best solution to work around this problem? I have an unused RCBS steel sizer die for the 45LC, which I tied lapping out. I spent an hour and only increased the diameter by about a thousandth. Not too promising as the die is super hard. I thought also, maybe a Lee factory crimp, or a Redding profile crimp die die might work on unsized cases, giving satisfactory bullet tension. Any guidence or suggestions would be appreciated.