The hillbillies who taught me groundhog and crow hunting always carried multiple boxes of handloads (with varying charge weights) when they went hunting because their .22-250 and .220 Swift loads all teetered on the brink of destruction. Melt, vaporize, sublimate, explode, ...call it what you will but it
almost always was marked by a telltale puff of gray smoke somewhere in mid-flight. I've seen it with my own two eyes dozens of times. When that happened, they'd switch to the next lighter load and go back to shooting.
In 2007, Eric Stecker, who became HMFIC of Berger Bullets when Walt Berger retired, did the engineer's due diligence and tested his own product to failure. He documented the firing of 950 6.5mm bullets from 1:8.5 twist .256 and .257 Krieger barrels into a berm at 1000 yards, with each shot witnessed by no fewer than two observers to confirm whether it reached the berm or failed in flight.
All told, 62 bullets failed in flight, beginning with the 106th shot fired.
Here is Eric's accounting of the testing.
Bullets fail in flight, that much is certain. But I don't think anyone ever has instrumented a bullet and then "failed" it, or photographed the disintegration process, so exactly what the mechanism of failure is is a matter of speculation and debate. One camp (which includes Eric Stecker) thinks the high temps soften the lead core to the plastic state, at which point the centripetal force pulls it apart. Some folks describe this as "melting" but that's an oversimplification to avoid having to tutor ignernt hillbillies like myself about the states of matter.
The other camp is equally as convinced that excessive RPMs are to blame.
But since you can't fire a bullet without both high temps
and high RPMs, I'm inclined to think it's a little of both, and either one could be the major culprit depending on all the particulars: ambient temperature, caliber, bearing surface length, rate of twist, # of grooves, type and depth of rifling, MV, rate of fire...
