Below is a Wilson case gauge with a case fired in a No.4 Enfield rifle, the amount it is sticking above the gauge is how much longer the military chamber is from a civilian SAAMI .303 British chamber. If you use full length resizing dies set up normally this is how much too far the sizing die will push the shoulder of the case back.
This is what will happen to your cases. (stretching in the base web area)
If you fire form your cases using a small rubber o-ring around the rim the o-ring when compressed will center the case in the rear of the chamber and hold the case against the bolt face.
Once fire formed if you neck size only the shoulder of the case will hold the case against the bolt face and your cases will not stretch and cause case head separations.
I fire form my cases using reduced loads and .312 pistol bullets.
By using this method I have gotten as many as 32 reloadings from a .303 case and the cases started failing due to neck cracks and not from case head separations.