With reloads I have had a few and seen a few centerfire case seperations (shooting .303's and .308's on the range does tend to give rise to overusage of brass) I have never even noticed until extraction, never seen any damage done at most a bit of inconvenience because a case extractor wasn't handy.
When it comes to ex mil ammo it is scarier. There is a reason most rifles have gas escape ports. Wear your glasses.
USUALLY (but not always) before it seperates you will see a crack start to develop just up from the base, this is very obvious with winchester brass sometimes after only three or four firings. And it is due to the brass thinning out. (minimum size brass and max chamber exacerbates it). I check with a piece of wire every time after I full length size.
This explains it well: from .303 British (Steve Redgewell), goto the link for the ful;l article with illustrations
http://www.303british.com/id29.htmlCase Head Separations
At the other end of the case, there can be a problem with separations. They are caused by a combination of events which leads to total case perforation near the head - this is the area just forward of the rim. (Fig. A - below) The perforation starts as a gradual thinning of the case wall. The thinning is caused by brass flowing forward from the head area upon firing. This creates a thin spot at the case head. (Fig. B - below)
Figure A.
Figure B.
Any case with a small shoulder angle, that is fired using maximum loads, will have brass flow. It moves toward the case mouth at a much greater rate than cases with more steeply angled shoulders. Improved or Ackley type cases with 35 or 40 degree shoulders are examples of brass with steep shoulders.
When you fire and full length resize your cases over and over, this aggravates the situation. Brass will continue to move forward. The weaker spots on the case are flexed, squeezed and contorted. Case necks become brittle.
Neck sizing reduces the occurance of seperations as the brass isn't worked as much.I have had more damage occur from neck splits and primer pocket/primer blowouts than I have ever had from seperations. Thankfully now that most of my milsurp is gone I hardly ever have any issue.