1. Commercial SAAMI .303 British cartridge cases are not made to British military standards.
2. The military Enfield chamber was reamed longer to make room for the mud of Flanders in WWI and does NOT match civilian commercial SAAMI standards.
When civilian commercial cartridge cases are fired in a military chamber with longer headspace settings you WILL get case stretching in the base web area.
The best surplus military ammunition you will find for reloading is Greek HXP ammunition. The best present day commercial cartridge cases are Prvi Partizan, they have thicker rims, a larger base diameter and thicker case walls.
Under normal conditions you could fire form your cases by seating your bullets long jamming into the rifling or making a false shoulder on the unfired case and headspacing on the shoulder. The problem is the Enfield rifle is long throated and seating your bullets long doesn't work and a false shoulder stresses the neck and can cause cracked necks.
Below is an old trick a Canadian taught me on fire forming cartridge cases for the military Enfield rifle. A rubber o-ring is slipped over the cartridge and pushed against the rim, the o-ring holds the case against the bolt face and the case will not stretch in the base web area when fired.
After fire forming your .303 cases you must neck size only or you will have case head separations, this allows the case to headspace on the shoulder and be held against the bolt face.
Below is a Wilson .303 case gauge and has a case inserted that was fired in a Enfield military chamber. The amount the cartridge is sticking above the gauge is how far the "military" chamber was reamed and lengthened in WWI and how "FAR" you will push the shoulder back if you full length resize your cases.
When fire forming cases I use .312 pistol bullets with reduced loads, I remove the extractor because the o-ring when compressed will center the case in the chamber and help promote equal case expansion and aid in accuracy.
Below and a note on headspace, military headspace is .064 to .074, with an American made cartridge case at maximum military headspace you can have as much as .016-.017 head clearance or "air space" between the bolt face and rear of the case. This is "why" your cases do not last long when reloaded, a Enfield forum member reached over 32 reloadings using the o-ring method of fire forming before he had is first case neck split. (NO case head separations)
With the Enfield below and several bolt heads I have set the headspace from .059 to .084 testing cases and stretching in the web area. It is far simpler to keep your existing bolt head and use the rubber o-ring than trying to find longer bolt heads that might not solve the problem and are getting more costly to find.
O-ring size will depend on headspace and type o-ring, lube your locking lugs to ease bolt closing and lube the o-ring so the o-ring will "crush and flow" and center your cases in the chamber. (American cases have small base diameters and can end up off center when chambered)
