The .303 British round headspaces from the rim of the cartridge, not the shoulder.
This means military rifles chambered for the round can, and often do, have large chambers of unusual dimensions. Despite that these rifles will function fine and shoot with decent military accuracy despite the chamber tolerance.
The idea of an over sized chamber actually has some good thought behind it, at least with rimmed rounds such as the .303. A chamber a bit over sized makes the rifle much less fussy about manufacturing variances in the ammunition. During a war time rush, this could be critical. Also consider, the British empire had armories on several continents at once, and tolerances sometimes had to be loose to make the ammo from colony 'A' fit the rifle from colony 'B'.
The .303 british cases shown are a military round (Greek HXP), a case fired from an Enfield No4MkII, and the same case ready to reload.
Notice the fired case compared to the unfired case. This pushing of the shoulder forward is caused by the case expanding to fill the chamber.
This rifle is not worn out. In fact, it appeared nearly unfired
when acquired. The chamber is normal for an Enfield of this type.
When shooting factory loaded
.303 ammunition, military or commercial, this large chamber makes no difference to the shooter. An Enfield owner who wishes to reload his cases had better pay attention to the phenomenon.
In resizing the case, the shooter has many choices.
If full length sizing, care must be taken regarding setting this shoulder back and overworking the brass. On the other hand, if more than one .303 rifle is owned, then the fired case will sometimes only fit that chamber and no other unless it's fully sized. Hand loading owners of .303's usually segregate their cases by rifle.
There is an opinion that the .303 is very hard on brass. Maybe, in some cases, it is. I believe the real culprit is not the rifle nor the cartridge, but the hand loading process. As for myself, my thirty year old .303 brass is on it's fifth or sixth loading with no issues apparent. Below-maximum pressure loads and careful sizing have given me .303 ammunition that functions well, is surprisingly accurate, and lasts as long as any other high power cartridge brass.
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