During WWI the chambers were lengthened on the Enfield rifle to make room for the mud of Flanders. The British military chamber does not match American SAAMI commercial chamber dimensions.
Below is a Wilson case gage which is nothing more than a SAAMI commercial chamber. The Wilson gages are normally used to adjust resizing dies for only pushing the shoulder of the case back .001 to .002. The fired .303 case below is resting on its shoulder and not the rim, the amount the case below is sticking above the gage is how much longer the British military chamber is than commercial standards.
The average rim thickness of American made .303 cases is .059-.058 this means at the maximum headspace setting of .074 you can have .016 to .017 head clearance.
Below a .303 case being fired, please note "headspace" and "head clearance" or the "air space" between the rear of the case and the bolt face and "why" the case stretches in the web area when fired.
Below is a 1950 No.4 Mk.2 that I experimented with adjusting the headspace from .010 over maximum (.084) to .005 under minimum (.059) Normal military headspace is .064 minimum to .074 maximum, commercial American headspace is .064 GO or minimum, .067 NO-GO or maximum and .070 to .071 as FIELD maximum.
Please note I have three .303 headspace gages but later I will explain how to check Enfield headspace "without" headspace gages.
Below is a once fired factory loaded Winchester .303 cartridge case cut in half after firing. This case was fired in an enfield with the headspace set just under .067 and it stretched .009 in the web area.
The factory loaded Winchester ammunition above only lasted three reloadings before it was tossed in the trash. So one thing you need to know if you reload is that American made commercial cases are "NOT" made to British military standards. They have smaller base diameters and the rims are thinner which causes stretching problems.
Below is a trick the Canadians taught me on how to fire form the .303 cartridge by slipping a rubber o-ring around the case forcing it into contact with the bolt face.
After fire forming with the o-ring the .303 case will headspace on the shoulder of the case and hold the case against the bolt face to help prevent stretching in the web area.
I normally fire form my .303 cases using .312 pistol bullets and a reduced load of Trail Boss.
NOTE: I remove the extractor form the bolt head when fire forming cases for two reasons, it keeps the extractor from cutting the o-ring. The second reason is I do not want the extractor interfering with the centering of the case as the bolt is closed. As the o-ring is compressed it centers the case in the rear of the chamber and promotes even case expansion. Remember you have a "fat" and "long" chamber and a case that runs on the small side in base diameter. So the o-ring serves two purposes. 1. Holding the case against the bolt face and 2. Centering the case in the large military chamber.
I no longer spend money on hard to find longer bolt heads, I just fire my cases to fit my chamber. One person in another Enfield forum reloaded his .303 case using the o-ring method 32 times before he had a split neck and "NO" case head separations.
. Makes reloadind easy. You only have to neck size, as long as you only shoot the one rifle with the ammo.
