Capt. Killingfield; On your reloading of the .303 British round,, the hot tip is to only Neck Size the cases.
Virtually all #4 Rifles have longer shoulders in the chambers which since the case headspaces on the rim allows the gun to chamber any round loaded anywhere in the Empire. The shoulder lengths do vary from country to country.
If you set the shoulder back once or twice it makes the case separate just in front of the Extractor groove, thus requiring the case removal tool.
Here's a pic of a factory loaded round and a reloaded case fired in my gun.
You can easily see the difference. I recommend a Lee Collet Neck Sizing Die as is squeezes the case neck down on a mandrel instead of shoving it into a sizing hole. This works the brass much less....