Rimmed cases like the .220 Swift can have separation at their pressure riing just like rimless ones do. It's usually caused by the full length sizing die set in the press too far down to size cases correctly for your rifle's chamber. That sets back the fired case shoulder too far and the next time its fired, the case body stretches forward. Repeated cycles of this thin the brass about 2/10ths inch in front of the rim from work hardening. It eventually cracks.
Solution is to take several fired cases, back the die up about 1/10th turn then full length size just one case case. Chamber that case and if the bolt does close easy on it, back the die up another 1/20th turn.
Then full length size another fired case. Test it for easy chambering in your rifle chamber.
If the bolt doesn't easily chamber on it, turn the die in 1/40th of a turn which is close to .002" difference in die height in the press, then skip to the next paragraph. Otherwise, turn the die counterclockwise and move it up 1/40th turn. and try again.
Then full length size another fired case. Test it for easy chambering in your rifle chamber. If it chambers easily, lock the ring on the die then leave it there. Repeat this die adjustment until a resized fired case just chambers easily.The die's now set to minimally set the fired case shoulder back.
One other way to make tiny but accurate adjustments to die position is the following:
http://s860.photobucket.com/user/jepp2/media/DieAdjustment-1.jpg.html
Print them on full size label paper then use the .002" one to stick on your sizing die's lock ring. Follow the directions to mark then adjust your die for precise settings.
Or buy an RCBS Precision Mic or Hornady LNL Case Gauge to measure your case headspace (head to shoulder reference) and adjust the die so fired cases are sized to set their shoulder back .002".