I once thought that these chambers were made with 375 reamers....not today. I've got a copy of the NRA hardback "Handloading" in the back is a list of cartride and CHAMBER drawings for the 375 and the 38-55. The chamebr should be .401 at the neck for the 375 and should be .3949 (.395) at the nec for the 38-55. If anything, the 38-55 has the tighter chamber. The brass should be .400 at the neck on the 375 and .3922 on the neck for the 38-55.
By these dimensions, if the chamber were true to 375 dimensions, 38-55 sized loads should FALL in, absolutely no resistance.
From this I think what is going on is we have 38-55 chambers, and 375 size loads. It is going to take a cerrosafe cast to find out for sure.
Another problem is with the length of the chambers and brass. The chamebr of the 38-55 should be 2.118 to the start of the throat, the 375 should be 2.080 If the chamber is a 375 chamber and hte brass is true to 38-55 length 2.129.-.020 it won't fit in a 375 chamber, it would stop against the throat and the action wouldn't close.
Others report that over time, Winchester has changed the length of their brass untill it is now all 2.080 regardless if it is 30-30, 375 or 38-55. If this is true, I can only surmise that oncvce the initial drawings are done on these cases, the same trim dies are used to reduce thier tooling costs. Anybody got some Winchester factory 38-55 brass that we can get a length on?
Haywire may be right on, we just need to have a custom reamer made to open our chambers by .002-.003" and be done with it. For my purposes, I don't mind screwing in my sizing die 2 turns and running the loaded cases back in, works fine, as I decap the case with a 45-70 die I'm using like a universal, so I don't have the decap stem in my sizing die and use it at any depth I like. I only turn it in 5 turns to size the brass after firing, I'm only neck sizing and I'm sure that cheatermk3 is right on, that one doesn't even need to size this brass after firing, certainly not with light loads anyway.