Recce21
10x just gave the best short explanation on case head separations and reloading the .303 British, head space and chamber diameter both effect how far your fired cases can stretch and thin in the web area.
I have been reloading for over 40 years and when a Canadian in another forum posted the the o-ring method for fire forming .303 cases I gave it a try and it is the only fire forming method I use for the .303 British now. The o-ring forces the case against the bolt face and the only direction the case case can expand and stretch is outward to meet the chamber walls.
Case construction, rim thickness, base diameter and the grade or type brass the case is made out of all effect case stretching in the base web area of the case and case head separations.
Some of you may know that the Greek HXP cases are highly rated for reloading and cases life, another even better case in my opinion are Prvi Partizan cases, which are easer to get and are newly made.
Below on the left is a once fired Prvi Partizan case and on the right is a once fired Greek HXP case, the Prvi Parizan case walls are thicker and the base diameter is larger than ANY currently made commercial .303 ammunition.
A chain is only as strong as its weakest link and American commercial ammunition is downloaded to the lower pressures levels of the early smokeless .303 ammunition which was approximately 38,000-40,000 CUP. This is to prevent firearms damage and lawsuits when commercial ammunition is fired in Enfield's made before the turn of the century.
When you reload this commercial ammunition the cases fall apart quickly when they are loaded to the max pressure levels of Mk.VII cartridge which is 46,000 CUP or 49,000 PSI. From reloading the Prvi Partizan cases I believe they are being made or manufactured to withstand the normal chamber pressure of 46,000 CUP (Mk.VII ammunition) and are the BEST cases for current reloading at higher chamber pressures without case head separations.
(The Prvi website states they sell military grade ammunition)
Recce21
Use the o-ring method of fireforming your cases and you will not have to guess at how much powder to use to fireform your cases OR worry about case thinning in the web area when fireforming.
P.S. DON'T use duct tape to fire form your cases.