I had a TRW M14 and an L1A1 and they don't produce cases wildly different from the loaded ammo. The millions of Garands out there also don't change the shape of their fired brass from the unfired ammunition. While the M14 didn't serve for long, the Garand and FAL did serve for decades and in a variety of different environments. They didn't seem to need overly loosey gooey chambers to run reliably.
You are speaking way too much in the modern sense. The 303 British case was designed in 1889, at a time when smokeless powder was in its infancy, heck, the first smokeless in the 303 British was Cordite, which is artillery powder.
To know why they had oversized chambers you have to look at the history of the Anglo-Zulu war, where the 577/450 cartridge of the Martini Henry was made of a copper foil rather than brass and was easily dented. These cases stuck in the rifles, to the point there guys were using knives and bayonets to try to pry them out while being killed by Zulu spears.
At no point in the history of the british military did then care at all about some guy reloading 100 years later, all they cared about was that the gun went bang when the guy pulled the trigger.
This is the same reason why the original M16 rifles were thrown into the jungle of Vietnam when they could get an M14, the gun and ammo needed to be sloppy to work under the conditions.
I do not suspect that the M14 and L1A1 would have worked out as well in the trenches of WWI as the No 1 Mk III Lee Enfield, the Ross rifle did not.
As a Lee Enfield shooter I want you to grab some ammo and head out to the range, enjoy your rifle for what it is and for as long as you can. They are beautiful guns.
One thing your posts are doing is reminding me that it has been a while since I pulled the trigger on my Enfields, they will be making the trip to the range with me on my next trip, thank you
