Forget the load for now and look at only two things -- (a) the fact that bullets are blowing-up and (b) the relatively low velocities for this to occur.
From these two facts, it seems obvious that your barrel is spinning those bullets too fast at velocities that seem too low for this to occur. In tests, for example, the Hornady 75-grain .224 A-Max was most accurate at about 292,000 rev/min and began coming apart at about 330,000 rev/min.
At 3,200 ft/sec,
� a 6-inch twist spins the bullet 384,000 rev/min. Too fast.
� a 7-inch twist spins the bullet 329,100 rev/min. Marginal at best. Most likely too fast for some bullets.
� an 8-inch twist spins the bullet 288,000 rev/min. Should be no problem with a good bore.
� a 9-inch twist spins the bullet 256,000 rev/min. Should be no prolem with a good bore.
At 3,300 ft/sec,
� a 6-inch twist spins the bullet 396,000 rev/min. Too fast.
� a 7-inch twist spins the bullet 339,400 rev/min. Too fast.
� an 8-inch twist spins the bullet 297,000 rev/min. Marginal
� a 9-inch twist spins the bullet 264,000 rev/min. Should be no problem.
The first probable culprit is the twist -- probably too fast.
Another probability is the surface of the bore -- probably scoring the jackets excessively.
Also possible is that the number of lands -- in conjunction with the above -- is just too many. The fewer the better.
And it may be also that the one brand of bullets has thinner, weaker jackets.
My latest .220 Howell rifle -- with my cartridge not greatly different from your cartridge -- can drive the 75-grain Hornady A-Max out the front door at 3,600 ft/sec (at moderate pressure) from its 9-inch twist (288,000 rev/min). This barrel has three grooves and has been lapped very smooth with a consistent diameter from chamber mouth to muzzle.
Your rifle is almost certainly spinning your bullets faster (shorter twist) and scoring their jackets more heavily. That's my best guess, based on what you've posted.
