I have been reloading for the 303 since the early 70s. It was designed to head space on the rim not the shoulder as rimless cases do. Head space that is tight to to tight is as bad as too loose. Been through the rim gauge stage with multable bolt heads.
I have had 3 case head seperations in the last 2 years second fireing, this was because the brass was of a very poor quality i dont think it would have made brass metal specs not with standing the under sized or minimum size of the case. Certainly smaller than military specs.
I also reload for a 243/303 @ 3800 fps, this rifle has never had a case head seperation plenty of neck shoulder splits if the annealing is not up to the mark.
The only factory loads i have used for years was some Highland (PRIVI)to try out the brass cases.
I also load for a barrel made in 1944 that could only be discribed as a match barrel yet it is on a service rifle with head space between 64 and 74.
These are SMLE and No4 actions.