What Happened?

That's why the best tool for anyone who reloads .303 is a broken case extractor. Because of the design of the web of the .303 cartridge case most reloaders are lucky to get 5 reloads out of a .303 cartridge case when they full length resize.
 
This is a classic case head separation. The primary cause is over sizing your brass relative to the headspace of the rifle it's fired in. Each time you size it down way less than the headspace you leave the cartridge no choice except to stretch. Eventually it fails above the web. This can be quite dangerous and give your face a blast of hot gas at 60,000 psi.

Take a fired case, back out your full lentgh sizing die and then adjust your sizing die so it just bumps the shoulder back a thou or so. Read up on headspace and you'll quickly figure it out.

BTW throw all your brass out, it'll fail soon. Take a dental pick and feel inside each case and you'll feel the web thinning inside the cases.
 
The .303 British cartridge headspaces on its rim, so in the military rifles the distance to the shoulder was not critical. When you resize the fired case and push back the shoulder, it means that the case blows out again. Do it a few times and the brass has stretched too much and you end up with the case head separation. Compare a new case, one of your reloaded cases and one that you have fired and you will see some noticeable differences in where the shoulder is.
The solution, as alluded to above, is to full length resize only enough to touch the shoulder on the fired case. The case head separation may still happen due to excessive stretching on the first firing, but if not, the partial sizing will prolong case life.
 
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