In all my years of repairing and shooting I have never seen a nipple split vertically [I have seen a few break horizonally, at a guess I would say you have gotten a hold of some that were harden to the point that they were brittle. I would suggest the "Uncle Mikes" or Hot Shot which are high enough quality to be consistant, there are some around that are made in certain countries that are very inconsistant in hardness. Lee