Headspace, (in a firearm, not a teenager) refers to the distance between the base of the cartridge and the head of the bolt. It can also refer to the index point on the cartridge that contacts the chamber, and controls how far into the chamber the round is inserted. With a rimless straight wall cartridge (.45 acp), the round usually headspaces on the front lip of the case. On a rimless bottleneck rifle cartridge (.223 ect) the cartidge headspaces on the shoulder.
The .303 British is a rimmed cartridge, and headspaces from the rim of the cartridge, not the shoulder. This is a carryover from an age when all rifle cartridges were rimmed, when positive extraction took precedence over other considerations. Developed during the 1880's, the.303 British is a venerable old design, but can be a bit persnickety at times.
Military rifles chambered for the round can, and often do, have large chambers of unusual dimensions. The chambers were often large on purpose, as fouling and dirt can close those dimensions in a hurry during military operations. 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'.
Military forces are not concerned with reloading their cases, with the possible exception of elite folks like the US Army Marksmanship Unit, where extreme accuracy is the stock in trade. For regular armed forces, it's going to be new made ammunition every time, and what happens to the cartridge when it's fired is not really a consideration, as long as it works and is safe to the soldier. This leaves the civilian hand loader with some interesting technical issues to deal with, especially with the .303 British fired from a military Lee Enfield.
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. (click on the photos to enlarge).
This rifle is not worn out. In fact, it appeared nearly unfired when acquired. The chamber is normal for an Enfield of this type. Case expansion in a Lee Enfield is not unusual, and therein lies the concerns for a hand loader.
When shooting factory loaded .303 ammunition, military or commercial, this large chamber makes no difference to the shooter. A Lee-Enfield owner who wishes to reload his cases had better pay attention to the phenomenon.
In resizing the case, the shooter has many choices. It's here the reloader can make or break his case life, and his accuracy, so choose wisely. When dealing with a rimmed cartridge like the .303, proper sizing can drastically effect case life and accuracy.
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. This applies to other rifles as well, but seems to really come into play with rimmed military rounds fired from military rifles. The Russian 7.62x54 is another prime example. Handloads that fit one rifle may not chamber in the next.
Perhaps the best policy, when loading rimmed cartridges to be used in elderly military rifles, is to segregate the cases by the rifle. Maintain a boxed set for each weapon, marked to the rifle by serial number or distinguishing factors. Then, reload those cases with the neck sizing method, or a full length sizing die which is backed off to leave the shoulder untouched. As long as the reloaded cartridge functions well, over working during sizing is not necessary and will shorten case life tremendously.
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.
Real talk: Why should judges bother studying that musty old Constitution anyway? - Might be time to up that prescription, buddy
17 minutes ago