Another thing to be aware of when loading for old .303 for Enfields is a problem with the head space of the gun. They are notorious for that. Case ruptures around the base/rim could be indicative of that. This is due to the bolt locking lug design. The locking lugs are located in the rear of the bolt, not in the front, like most other rifles of the class. In older rifles, repeated firing can actually compress the bolt body minutely or stretch the action enough to cause excessive head space. This is a very dangerous condition, and can and will cause catastrophic case failure around the base, venting the hot gas into the shooter's face and and/or blowing the action up.
If you have another case rupture, have it checked by a qualified gunsmith, if not before. The problem can be easily fixed usually, by head space adjustment. However, not after you blow it up.