“but will it really make a significant difference in the M1?”
First, I would suggest a reloader that owns a press, die and shell holder learn to use it, then I would suggest the same reloader learn to deterring the length of the chamber from the shoulder of the chamber to the bolt face. There is a sequence of events that are repeated over and over on the rleoading forums, stats something like this:
The firing pin strikes the primer, then everything including the case, powder bullet and primer runs to the front of the chamber when it slams into the shoulder of the chamber etc., etc.. Then the sequence of events ends with case head stretch between the case head and case body. All of the above would be true if all receiver designs were the same.
Problem with the M1 Garand, hot, high pressure, metal cutting gas escaping from around the primer when the case head separates from the bolt face. I know, neck size, sounds good, but to get to neck sizing cases for a chamber the case must first be fired in the chamber. Gas cutting, a bolt face can experience gas cutting with one mishap.
Mixed cases, I do not have mixed cases, I am the fan of mixing case head stamps in the case cleaning tumbler then but all the cases back into the box they came in.
F. Guffey