Perry - I'm really happen this has come together for you and you've managed to be able to shoot the gun some.

It's really great the barrel isn't picking up an copper to speak of as well. That is a BIG plus in itself.

Now, some observations and suggestions - judging by the looks of the brass and the amount of difference between unfired and fireformed brass, I'd be giving thought to opening up the necks of new, undfired brass to .375, then necking them down to make a crush fit before firing any more. the fasle shoudler produced by necking to .375 should work just fine with such a small diameter case. If larger diameter is needed, you could quite literally make perfectly fitting brass prior to fireforming if they are necked to .40 calibre first, which is quite straight for a .303 case, then use the die to position the final shoudler exactly where it should be as you size the neck down for .358 bullet and crush fit on chambering.

I am quite concerned as-is, about your brass life due to the mount the shoulder has actually moved foreward. This means the case has stretched at the web on the initial firing. Even .005" is too much for me - the brass has stretched. It might not be enough to feel yet with a bent wire, but is must have stretched.

All that is necessary, is to use something like a .375 Winchester neck expander die, or merely a .375 expander button on your die's threaded shaft. Just screw it down to sticking out the bottom of the die, lube the .303 case and run it up over the button then back it off. Set aside as you do the rest, then change bottons, have the gun handy, and run them into the die, set long, then adjust to get a crush fit on your necksized brass. Only screw the die down 1/4 turn at a time until you can close the bolt with stiff feeling. Nekcing up from .303 to .375 is not too big a step, but if you feel it is, open them to .358 first, then .375. I go from .308" to .430" or .45" in one step - on '06 brass or 8x57 new brass for my .375 wildcat and 9.3x62 or 9.3x57. I've not lost a case yet in doing so with new brass. It is soft enough to 'take the gaff'.

If they are fit to a crush fit as you close the bolt, there will be NO stretching at the web when they are fired andthe case will form perfectly to the chamber. Any stretch at the web will continue each time they are fired until the case separates at the web - common with the parent (.303) case in 1 to 4 firings depending on the rifle and chamber dimensions.

If necked up first to establish a false shoulder as described, with the resulting no stretch at the web and then neck sizing thereafter, the brass should last until you are tired of using the same old cases- of course, trimming and annealing as necessary.