Originally Posted by Catfur
Berger went to increasing the thickness on their match bullets because of in-flight failure during match conditions, where long strings caused intense barrel heat. The hot barrel would transfer enough heat to the lead core to cause it to melt, and the jacket can't contain the liquid (with no structural integrity of it's own). Since the bullet failures occurred in the hottest part of the match, towards the end of a string, this may be the cause.
Given that it is the only theory with such compelling supporting data (though in a different bullet design), I'd have to say this idea is a leading contender at this point.
However, this theory says very little about why we never saw the problem shooting hundreds of bullets and numerous matches in Colorado. Yes, it was usually 15 degrees or more cooler in Colorado, but is that difference in ambient temperature really enough? Maybe, but gotta keep an open mind about other causes.