reloading 303.

[ireload2]

You might take a look at the shoulder of a number of your fired cases and compare them with a new cartridge. You will find the LE chambers vary a lot in shape from each other. They also vary a LOT from the shape of a new unfired cartridge. This presents a challenge since reloading dies normally produce a sized case that more closely resembles the unfired rounds instead of the various chambers. The best compromise might be to find the smallest chamber you have and resize to fit it knowing that the loads will fit anything with a larger chamber.
A learning exercise you might try is take a box of 20 and reload and shoot then until a few fail. Depending on the circumstances of your brass, chamber, loads and loading techniques you will probably get a few failures before you have reloaded the box 5 times. Wear glasses. To prepare for the inevitable you might take a rough fired case or two and trim most or all the neck off and keep them with you when you go to the range. When you have a case head separation you can run the neckless case into the shell that is left in the chamber and bump the bolt into it. Then retract the bolt and it will pull out the headless case. Once you have seen the way fired brass behaves you can try fine tuning your technique to extend the life of your brass. I am am sure you will get plenty of help here.