In 1914 the military .303 chamber was enlarged to make room for the mud of Flanders to ensure function under harsh conditions. This means the chamber is not even close to American commercial SAAMI standards in length and diameter.
Below is a Wilson case gauge and a unfired surplus South African .303 cartridge, please notice the case is even with the top of the gauge.
Below is the same cartridge after it has been fired, the amount the case is sticking above the gauge is how much longer the military chamber is than a commercial chamber.
If you full length resize your .303 cases you will be pushing the shoulder of the case back over 1/8 of an inch too far and will be "OVER RESIZING" your cases and cause case head separations.
Please remember this, your resizing dies are much smaller than the military chamber in length and diameter so you MUST compensate for the difference for the smaller resizing dies.
Below is a animated image of a commercial .303 British cartridge case being fired in a military chamber. Please note the words "headspace" and "head clearance" and how the case will stretch to meet the bolt face.
I place a small rubber o-ring around the case for fire forming the cases, the o-ring holds the case against the bolt face and keeps the case from stretching. Also when the o-ring is compressed it centers the case in the rear of the chamber and helps promote accuracy.
Below the o-ring holding the case against the bolt face.
After fire forming your cases the shoulder of the case will hold the case against the bolt face just like a standard rimless case. WARNING, neck size only and if you must bump the shoulder back buy a .303 case forming and trim die and use it as a shoulder bump die.
Most of my cases are fire formed using light loads and 100 grain .312 Hornady pistol bullets.
Using this fire forming method I have gotten over 30 reloads from my .303 British cases.